Lexical Definition

‎νήπιος‎ nḗpios (nay'-pee-os) child ^G:A
from an obsolete particle (implying negation) and ‎ἔπος‎

babe, child (+ -ish).

• not speaking, i.e. an infant (minor)
• figuratively, a simple-minded person, an immature Christian

Abbott-Smith

νήπιος, -α, -ον, [in LXX chiefly for עוֹלֵל, also for פְּתִי, etc.;]
infant; of children and minors: Mt 21:16(LXX), I Co 13:11, Ga 4:1 (v. Lft., in l.). Metaph., childish, unskilled, simple (Ps 18 (19):8*, Pr 1:32, al.): Mt 11:25, Lk 10:21, Ro 2:20, Ga 4:3, Eph 4:14, I Th 2:7 (WH, fox ἤπιοι); opp. to τέλειος, He 5:13; ν. ἐν Χριστῷ, I Co 3:1.†

Abbott-Smith References

Liddell-Scott-Jones

νήπιος, α

[Refs 2nd c.AD+], Ionic dialect η, ον, also ος, ον [Refs 3rd c.BC+]:— infant, child, frequently in [Refs 8th c.BC+]; ἐκ νηπίου from a child, from infancy, [τὸ ἡδὺ] ἐκ ν. ἡμῖν συντέθραπται [Refs 2nd c.BC+]; infant in law, minor, ἐφ᾽ ὅσον ὁ κληρονόμος ν. ἐστιν [NT]; of children up to puberty, αἱ τῶν ν. ἐκλάμψιες [Refs 5th c.BC+]; but of the foetus in its early stage, [Refs 5th c.BC+]

__2 less frequently of animals, [Refs 8th c.BC+]; νήπια alone, the young of an animal, [Refs]
__3 of plants, [Refs 4th c.BC+]
__II metaphorically,
__II.1 of the understanding, childish, silly, [Refs 8th c.BC+]; simply, without foresight, blind, [Refs 8th c.BC+]; οὔτε πρὶν νήπιον, νῦν τ᾽.. μέγαν no child before and now full-grown (i.e. in mind), [Refs]; of words, νήπια βάζεις [Refs 5th c.BC+]
__II.2 of bodily strength, like that of a child, βίη δέ τε ν. αὐτῶν [Refs 8th c.BC+]

_{LSJ Scripture Refs: Gal.4.1}