Probably from meno; remaining, i.e. Sole or single; by implication, mere -- alone, only, by themselves.
see GREEK meno
1. an adjective, alone (without a companion);
a. with verbs: εἶναι, ἑυρίσκεσθαι, καταλείπεσθαι, etc., Matthew 14:23; Mark 6:47; Luke 9:36; John 8:9; 1 Thessalonians 3:1; added to the pronouns ἐγώ, αὐτός, οὐ, etc.: Matthew 18:15; Mark 9:2; Luke 24:18; Romans 11:3; Romans 16:4, etc.
b. it is joined with its noun to other verbs also, so that what is predicated may be declared to apply to some one person alone (cf. Winer's Grammar, 131 (124) note): Matthew 4:10; Luke 4:8; Luke 24:12 (T omits; L Tr brackets WH reject the verse); John 6:22; Hebrews 9:7; 2 Timothy 4:11; with a neg. followed by ἀλλά, Matthew 4:4. ὁ μόνος Θεός, he who alone is God: John 5:44; John 17:3; Romans 16:27; ὁ μόνος δεσπότης, Jude 1:4. οὐκ ... εἰ μή μόνος: Matthew 12:4; Matthew 17:8; Matthew 24:36; Luke 6:4; οὐδείς ... εἰ μή μόνος, Philippians 4:15. equivalent to forsaken, destitute of help, Luke 10:40; John 8:16; John 16:32, (Wis. 10:1).
2. Neuter μόνον as adverb, alone, only, merely: added to the object, Matthew 5:47; Matthew 10:42; Acts 18:25; Galatians 3:2; to the genitive Romans 3:29 (here WH marginal reading μόνων); referring to an action expressed by a verb, Matthew 9:21; Matthew 14:36; Mark 5:36; Luke 8:50; Acts 8:16; 1 Corinthians 15:19; Galatians 1:23; Galatians 2:10. μόνον μή, Galatians 5:13; οὐ (μή) μόνον, Galatians 4:18; James 1:22; James 2:24; followed by ἀλλά, Acts 19:26 (L ἀλλά καί; cf. Winers Grammar, 498 (464); Buttmann, 370 (317)); by ἀλλά πολλῷ μᾶλλον, Philippians 2:12; by ἀλλά καί, Matthew 21:21; John 5:18; John 11:52; John 12:9; John 13:9; John 17:20; Acts 19:26 (Lachmann (see as above, especially Buttmann));