The Hebrew word בְּמֹתָיו literally translates into English as “by/ in/ with his deaths.”
Declension of מָוֶת
The lemma or lexical form is מָוֶת (mā'wet). The singular construct form is מוֹת (mot), meaning “death of.” The plural absolute form is מוֹתִים (mô'tîm), meaning “deaths.” The plural construct form is מוֹתֵי (mô'tê), meaning “deaths of.”
To the aforementioned construct forms may be affixed pronominal suffixes. For example, adding a 3rd person, masculine gender, singular number pronominal suffix to the singular construct form מוֹת yields מוֹתוֹ, meaning “his/its death.” Adding a 3rd person, masculine gender, singular number pronominal suffix to the plural construct form מוֹתֵי yields מוֹתָיו, meaning “his/its deaths.” This is the form that occurs in Isa. 53:9, with the addition of the prepositional prefix בְּ.
The plural declension of מָוֶת only occurs elsewhere besides Isa. 53:9 in Eze. 28:10 by means of the construct form מוֹתֵי, “deaths of”:1
10 “You shall die the deaths of the uncircumcised by the hand of strangers, for I have spoken it,” said Yahveh God.
י מוֹתֵי עֲרֵלִים תָּמוּת בְּיַד זָרִים כִּי אֲנִי דִבַּרְתִּי נְאֻם אֲדֹנָי יַהְוֶה
It is noteworthy that the subject, the prince of Tyre,2 is said to “die the deaths (plural) of the uncircumcised.” Thus, like Isa. 53:9, there is a singular individual who is said to die plural deaths.
In his commentary on Eze. 28:10, Carl Friedrich Keil wrote,3
The plural מְמוֹתֵי and מוֹתֵי here and Jer. xvi. 4 (mortes) is a pluralis exaggerativus, a death so painful as to be equivalent to dying many times (see the comm. on Isa. liii. 9).4
Similarly, in his commentary on Isa. 53:9, Franz Delitzsch wrote,5
“They assigned Him His grave with criminals, and after He had actually died a martyr's death, with a rich man;” i.e., He was to have lain where the bodies of criminals lie, but He was really laid in a grave that was intended for the corpse of a rich man.”
מותי is a plur. exaggerativus here, as in Ezekiel 28:10 (compare (memōthē) in Ezekiel 28:8 and Jeremiah 16:4); it is applied to a violent death, the very pain of which makes it like dying again and again. The first clause states with whom they at first assigned Him His grave; the second with whom it was assigned Him, after He had really died a painful death.
Delitzsch, Franz. Commentary on the Old Testament. 1900. Reprint. Trans. Martin, James. Vol. 7. Grand Rapids: Eerdmans, 1986.
Keil, Carl Friedrich. Commentary on the Old Testament. 1900. Reprint. Trans. Easton, M. G.; Martin, James. Vol. 9. Grand Rapids: Eerdmans, 1988.
1 A synonym of similar spelling, מָמוֹת (mā'môt), also occurs twice in the Tanakh. Coincidentally, it also occurs in Eze. 28:8 wherein the same subject, the prince of Tyre, is again said to die plural deaths: וָמַתָּה מְמֹותֵי חָלָל בְּלֵב יַמִּים (“...and you shall die [the] deaths of one slain in the heart of the seas.”).
2 Eze. 28:2
3 p. 408
4 It is noteworthy that the LXX translates the Hebrew of Isa. 53:9 into Greek as «τοῦ θανάτου αὐτοῦ» (“his death”) and Eze. 28:8 as «θανάτῳ» (“by the death”). (It does not appear to translate Eze. 28:10 word-for-word.)
5 p. 328
6 ibid, p. 329