Sense

1. naked, bare
2. bald, smooth
3. unclad, uncovered
4. small, frail, delicate
5. (military) light (troops)
6. unarmed
7. (of words) without meter (i.e. prose)
8. (poetry) without music (Epic vs Lyrical)
9. (singing) without music (a capella)
10. (music) without singing (instrumentals)
11. (grammar) without the rough breathing (i.e. with the smooth breathing)
12. (grammar) describing the unaspirated voiceless stops, π ‎(p), τ ‎(t), κ ‎(k), as opposed to the aspirated voiceless stops, φ ‎(ph), χ ‎(kh), θ ‎(th)

Examples

• ψιλὴ γάρ τισ ἡ γραφὴ τῶν λόγων. (Lucian, De Domo, (no name) 21:11)
• καὶ τὰ ἄλφιτα οὐκέτ’ ὀλίγα οὐδὲ ὡσ πρὸ τοῦ μᾶζα ψιλή, τὸ δὲ ὄψον οὐ τάριχοσ ἢ θύμον, ἀλλὰ κρέα παντοδαπὰ καὶ οἶνοσ οἱο͂σ ἥδιστοσ, καὶ χρυσίον παρ’ ὅτου ἂν ἐθέλωσι· (Lucian, Fugitivi, (no name) 14:4)
• ὅπερ οὖν ἔφην, οὐ δύναται ψιλὴ λέξισ ὁμοία γενέσθαι τῇ ἐμμέτρῳ καὶ ἐμμελεῖ, ἐὰν μὴ περιέχῃ μέτρα καὶ ῥυθμούσ τινασ ἐγκατατεταγμένουσ ἀδήλωσ. (Dionysius of Halicarnassus, De Compositione Verborum, chapter 2515)
• καὶ μηδεὶσ ὑπολάβῃ θαυμαστὸν εἶναι τὸν λόγον, εἰ καὶ τῇ πεζῇ λέξει φημὶ δεῖν ἐμμελείασ καὶ εὐρυθμίασ καὶ μεταβολῶν, ὥσπερ ταῖσ ᾠδαῖσ καὶ τοῖσ ὀργάνοισ, εἰ μηδενὸσ τούτων ἀντιλαμβάνεται τῆσ Δημοσθένουσ ἀκούων λέξεωσ, μηδὲ κακουργεῖν με ὑπολάβῃ τὰ προσόντα τῇ ψιλῇ λέξει προσμαρτυροῦντα. (Dionysius of Halicarnassus, De Demosthene, chapter 48 3:4)
• ταῦτα δὲ χρὴ ἐνθυμεῖσθαι ὡσ κάλλιστα καὶ τῶν ὑδάτων πέρι ὡσ ἔχουσι, καὶ πότερον ἑλώδεσι χρέονται καὶ μαλθακοῖσιν ἢ σκληροῖσί τε καὶ ἐκ μετεώρων καὶ πετρωδέων εἴτε ἁλυκοῖσι καὶ ἀτεράμνοισιν‧ καὶ τὴν γῆν, πότερον ψιλή τε καὶ ἄνυδροσ ἢ δασεῖα καὶ ἔφυδροσ καὶ εἴτε ἔγκοιλόσ ἐστι καὶ πνιγηρὴ εἴτε μετέωροσ καὶ ψυχρή‧ καὶ τὴν δίαιταν τῶν ἀνθρώπων, ὁκοίῃ ἥδονται, πότερον φιλοπόται καὶ ἀριστηταὶ καὶ ἀταλαίπωροι ἢ φιλογυμνασταί τε καὶ φιλόπονοι καὶ ἐδωδοὶ καὶ ἄποτοι. (Hippocrates, Hippocrates Collected Works I, , i.5)

Source: Ancient Greek entries from Wiktionary