οἰκητήριον is just habitation, or dwelling place. It's not a difficult word to interpret, and translations like "first estate" are excessively flowery for it. That does not mean that the theological interpretation is simple.
In Jude, this is a reference to the well known jewish tradition that angels descended to earth to have sex with women. This tradition is present in many manuscripts but the most famous would be the inter-testamental pseudoepigraphia that was widely circulated. Most commentators believe Jude is citing 1 Enoch and/or the Testament of Naphtali here.
Here is the NAC commentary:
Before providing more detail on Jewish tradition, it would be helpful
to explain what Jude said in v. 6. He charged the angels with not
keeping “their positions of authority.” The Greek word here is archēn,
signifying the domain or rule or sphere of influence given to the
angels. The angels abandoned “their own home” (to idion oikētērion)
and transgressed proper bounds. The language is rather vague. What
Jude meant, however, was that they left their proper sphere, came to
the earth, became males, and had sexual relations with women. Jude
used the language of retaliation here. Since the angels “did not keep”
(mē tērēsantas) their proper sphere, God “has kept” (tetērēken) them
“in darkness.” Abandoning what is right has consequences because God
is still Lord of the world. These angels experience punishment even
now in that they are “bound with everlasting chains.” We might think
that literal chains are in view, but Hillyer rightly remarks: “We are
not intended to imagine a literal dungeon in which fallen angels are
fettered. Rather, Jude was vividly depicting the misery of their
conditions. Free spirits and celestial powers, as once they were, are
now shackled and impotent. Shining ones, once enjoying the marvelous
light of God’s glorious presence, are now plunged in profound
darkness.” Their current imprisonment, however, is not their final
punishment. They are being preserved even now for the judgment on the
day of the Lord. Now they are imprisoned, but they still await their
final and definitive judgment on the last day.19 The main point is
that those who transgress and sin will experience judgment. The angels
did not escape unscathed when they violated what was fitting. Neither
will the opponents sin with impunity, and hence Jude encouraged the
church to resist their teaching.
Schreiner, T. R. (2003). 1, 2 Peter, Jude (Vol. 37, pp. 448–449). Nashville: Broadman & Holman Publishers.
Many believe that Jude is specifically referring to 1 Enoch since it has the most detail on this -- these angels are called Watchers, and there is a whole book about them in 1 Enoch -- and also because Jude directly cites 1 Enoch in verse 14:
"And Enoch also, the seventh from Adam, prophesied of these, saying,
Behold, the Lord cometh with ten thousands of his saints, "To execute
judgment upon all, and to convince all that are ungodly among them of
all their ungodly deeds which they have ungodly committed, and of all
their hard speeches, which ungodly sinners have spoken against him."
NAC provides a good summary of the tradition Jude refers to:
The tradition, as we said, is most extensive in 1 Enoch. The angels
desired the daughters of men (6:1–2) and took them as wives, who in
turn gave birth to giants (7:1–2; 9:7–9; 106:14–15, 17). As a result
of their sin, God threatened to send a flood (10:2). The evil of the
angels is quite clear when the author said they “fornicated” with
women (10:11). Some of the language used bears remarkable parallels to
Jude. The angel Raphael is ordered to “ ‘Bind Azaz’el hand and foot
(and) throw him into the darkness!’ And he made a hole in the desert
which was in Duda’el and cast him there; he threw on top of him rugged
and sharp rocks. And he covered his face in order that he may not see
light; and in order that he may be sent into the fire on the great day
of judgment” (10:4–6). Jude also taught that the angels who sinned
were bound in darkness and await the day of judgment. That those who
sinned will experience a temporary judgment before the final judgment
is clearly communicated in 1 Enoch 10:12–13: “Bind them for seventy
generations underneath the rocks of the ground until the day of their
judgment and of their consummation, until the eternal judgment is
concluded. In those days they will lead them into the bottom of the
fire—and in torment—in the prison (where) they will be locked up
forever” (cf. 13:2). Similarly, the Watchers are told, “You will not
be able to ascend into heaven unto all eternity, but you shall remain
inside the earth, imprisoned all the days of eternity” (14:5; cf.
21:1–4, 10; 88:1, 3). The idea that the Watchers abandoned their
proper sphere, emphasized in Jude, is communicated in 1 Enoch as well
(along with a concise summary of the event): “For what reason have you
abandoned the high, holy, and eternal heaven; and slept with women and
defiled yourselves with the daughters of the people, taking wives,
acting like the children of the earth, and begetting giant sons?”
(15:3). Jude followed the tradition in pronouncing judgment on angels
who violated their proper sphere.
Schreiner, T. R. (2003). 1, 2 Peter, Jude (Vol. 37, pp. 449–450). Nashville: Broadman & Holman Publishers.
As an added, bonus, even though there are only Greek fragments extant of 1 Enoch (The complete text is available only in Ethopian, as the Ethiopian church is the only church to hold that Enoch is canonical) we do find the requested οἰκτήριον in the Greek text:
Then answered Sariel1, one of the holy angels who was with me, and
said to me, “This cursed valley2 is for those who are cursed forever.
Here will be gathered all the cursed, who utter with their mouth an
improper word against the Lord and speak hard things against his
glory. Here they will be gathered, and here will be (their)
Nickelsburg, G. W. E. (2001). 1 Enoch: a commentary on the book of 1 Enoch. (K. Baltzer, Ed.) (p. 317). Minneapolis, MN: Fortress.
1 Could also be 'Uriel'
2 Valley of Hinnon
3 οἰκτήριον in the Akhmim papyrus of Greek Enoch (Codex Panopolitanus)
So putting this all together, the habitation that the Angels left was heaven/their spiritual bodies, so that they could be on earth and have carnal bodies in order to seduce women, and so God gave them an even lower habitation in the gloomy darkness. This provides a nice contrast with 2 Cor, where we yearn to put on the spiritual bodies. The use of habitation is simple, but the theology of angels putting on flesh and human flesh being swallowed in spirit is not.