The definition of λῃστῶν is:
λῃστής, οῦ, ὁ (ληϊς, epic form of λεία ‘booty, spoils’; Soph., Hdt.+; ins, pap, LXX; ApcSed 15:3; Joseph.; loanw. in rabb.; Ar. 3, 2;
Just., Tat., Ath., R. 19 p. 72, 25; Theoph. Ant. 3, 14 [p. 232, 13]).
① robber, highwayman, bandit (in Palestine: Jos., Bell. 2, 125;
228 al.) Lk 10:30, 36; 2 Cor 11:26 (Chariton 6, 4, 6 λῃσταῖς θαλάττῃ);
Mt 26:55; Mk 14:48; Lk 22:52; so also MPol 7:1. Crucified w. Christ Mt
27:38, 44; Mk 15:27. W. κλέπτης (Pla., Rep. 351c; Ep. 63 of Apollonius
of Tyana [Philostrat. I 363, 21]) J 10:1, 8. σπήλαιον λῃστῶν a
bandits’ cave or hideout (Jer 7:11) Mt 21:13; Mk 11:17; Lk 19:46; 2 Cl
14:1 (GBuchanan, HUCA 30, ’59, 169–77: ‘cave of brigands’; s. ἱερόν b,
end; Schürer II 600).—This mng. was extended to signify
② revolutionary, insurrectionist, guerrilla (Jos., Bell. 2,
254=σικάριος; 253; 4, 504, Ant. 14, 159f; 20, 160f; 167) of Barabbas
(cp. μετὰ τ. στασιαστῶν Mk 15:7) J 18:40 (HRigg, Jr., JBL 64, ’45, 444
n. 95; HWood, NTS 2, ’55/56, 262–66 and JTwomey, Scripture (Edinburgh)
8, ’56, 115–19 support this, but see MHengel, Die Zeloten, ’61, 25–47;
344–48); prob. also in the words of Jesus Mt 26:55; Mk 14:48; Lk
22:52; MPol 7:1 (cp. Mt 26:55).—More precise def. depends on
assessment of ‘social banditry’, s. RHorsley, Josephus and the
Bandits: Journal for the Study of Judaism 10, ’79, 37–63;
RHorsley/JHanson, Bandits, Prophets, and Messiahs ’85.—B. 791. DELG
s.v. λεία. M-M. TW. Spicq.1
Of the 15 occurrences of λῃστής in the New Testament, all major translations render the word as "robber" or "theif" in most cases and this seems to be the clear meaning in each instance. The word is a Greek translation of the Hebrew word פָּרִיץ (this is a direct quote from Jeremiah 7:11), which means "brigand, thief, [or] robber."2
The word σπήλαιον is defined as:
σπήλαιον, ου, τό (Pla. et al.; Sb 5295, 7; LXX, TestSol, Just.) prim. ‘cave’; as a σπήλαιον λῃστῶν, a robbers’ hideout (Jer 7:11;
cp. Jos., Ant. 14, 415; 421; Field, Notes 15) Mt 21:13; Mk 11:17; Lk
19:46 (s. on ἱερόν b): 2 Cl 14:1. As a place of refuge (Cornutus 27 p.
50, 5; Jos., C. Ap. 1, 292; 300) B 11:4 (Is 33:16); Hb 11:38; Rv 6:15.
As a place of birth GJs 18:1; 19:1 (codd.), 2f; 20:4; 21:3 (Just., D.
78, 5). Of tombs (TestReub 7:2; TestIss 7:8; ViEzk 4 [p. 74, 10 Sch.];
ViDan 20 [p. 79, 11 Sch.]) J 11:38.—HLavagne, Operosa Antra,
Recherches sur la grotte à Rome de Sylla à Hadrien ’88.—DELG. M-M.
This phrase (σπήλαιον λῃστῶν, and its corresponding Hebrew construction: מְעָרַת פָּרִצִים) is probably best translated as "robbers' hideout" (maintaining the genitive plural construction of λῃστής; note the plural placement of the possessive apostrophe), and is verbatim from the Septuagint (LXX) translation of Jeremiah 7:11.
The IVP commentary points out:
Jesus cites two texts as the basis for his attack. The first is Isaiah
56:7. God’s house was intended to be for all peoples (1 Kings 8:41–43;
Is 56:7), and in the Old Testament the only separation in the temple
was between priests and people. But in Jesus’ day the temple was also
segregated by race and gender for purity reasons, with Jewish women on
a lower level outside the Court of Israel and non-Jews in the
outermost court. Jesus shows his concern for the worship of the
Gentiles and protests racial segregation in a religious institution.
The second text he cites is from Jeremiah 7:11, where Jeremiah
condemns the idea that the temple is a safe haven for Judah in its
sin; although those who have exploited the poor think that the temple
will protect them, God will destroy his temple (Jer 7:3–15). Robbers’
“dens” were where robbers kept their loot; in A.D. 66 rebel brigands
or “robbers” (for whom Josephus uses the same term as Mark) took
possession of the temple and slaughtered the priests, further inviting
God’s impending wrath....4
The NET translators offer some commentary as to the possible meaning of this term in this context:
The meaning of Jesus’ statement about making the temple courts a den
of robbers probably operates here at two levels. Not only were the
religious leaders robbing the people financially, but because of this
they had also robbed them spiritually by stealing from them the
opportunity to come to know God genuinely. It is possible that these
merchants had recently been moved to this location for
1 William Arndt, Frederick W. Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature, 3rd ed. (Chicago: University of Chicago Press, 2000), 594.
2 Ludwig Koehler, Walter Baumgartner, M. E. J. Richardson and Johann Jakob Stamm, The Hebrew and Aramaic Lexicon of the Old Testament, electronic ed. (Leiden; New York: E.J. Brill, 1999), 968.
3 Arndt, Danker, and Bauer, 938.
4 Craig S. Keener, The IVP Bible Background Commentary: New Testament (Downers Grove, IL: InterVarsity Press, 1993), Mk 11:17.
5 Biblical Studies Press, The NET Bible First Edition Notes (Biblical Studies Press, 2006), Mk 11:17.