Lexical Definition

‎πρηνής‎ prēnḗs (pray-nace') headlong ^G:A
from ‎πρό‎

headlong.

• leaning (falling) forward ("prone"), i.e. head foremost

Abbott-Smith

** πρηνής, -ές (in Att. also πρανής), [in LXX: Wi 4:19, III Mac 5:43, 50 6:23*;]
headlong, prone: Ac 1:18.†

Abbott-Smith References

Liddell-Scott-Jones

πρηνής-ής, ές,

Ionic dialect πρηνής (also in [Refs 4th c.BC+], genitive έος, Attic dialect contraction οῦς:—of posture, with the face downwards, lying on the front, falling forwards, opposed to ὕπτιος, πρηνεῖς τε καὶ ὕπτιοι ἔκπεσον ἵππων [Refs 8th c.BC+]; πρηνέα.. τανύσσας [Ἕκτορα[Refs]headlong down, [NT] (perhaps ={πρησθείς} becoming distended); ἐπὶ τὸ πρηνὲς ῥέπειν incline towards pronation, [Refs 5th c.BC+]; of the arm and hand, with the palm downwards, variant in [Refs]; opposed to ὕπτιος, [Refs 4th c.BC+]; of ἀστράγαλοι, ὀρθοὶ πίπτοντες ἢ πρηνεῖς [Refs 2nd c.AD+]; of seeds, hollow side downwards, [Refs 4th c.BC+]; of a ship, bottom upwards, implied in [Refs 1st c.AD+]

__II of parts of animals or man, that part which is uppermost and visible when the animal or man is in the πρανής position (the normal one for a quadruped), the back part, τὰ τετράποδα.. ἐν τοῖς ὑπτίοις οὐκ ἔ χει τὰς τρίχας, ἀλλ᾽ ἐν τοῖς πρανέσι μᾶλλον· οἱ δ᾽ ἄνθρωποι τοὐναντίον ἐν τοῖς ὑπτίοις μᾶλλον ἢ ἐν τοῖς πρανέσιν [Refs 4th c.BC+]
__II.2 of leaves and of the hand, the back or 'wrong' side, τὰς ἶνας καὶ τὰς φλέβας ἐν τοῖς π. ἔχουσιν ὥσπερ ἡ χείρ [Refs 4th c.BC+] (misunderstood as the opposite by [Refs 1st c.AD+]
__III of the sides of hills, πρὸς ἄναντες καὶ κατὰ πρανοῦς καὶ πλάγια ἐλαύνειν down hill, [Refs 5th c.BC+]; τὸ π., opposed to τὸ ὄρθιον, [Refs]
__III.2 convex, [Refs 4th c.BC+]

_{LSJ Scripture Refs: Acts.1.18}