Etymology
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immature (adj.)
1540s, "untimely, premature," from Latin immaturus "untimely, unripe," from assimilated form of in- "not, opposite of" (see in- (1)) + maturus "ripe, timely, early" (see mature (v.)). In 16c., usually in reference to early death; main modern sense of "not fully developed" first recorded 1640s. In reference to mentalities or behaviors not considered age-appropriate, from 1920. Related: Immaturely.
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immaturity (n.)
1530s, "untimeliness," from Latin immaturitatem (nominative immaturitas) "unripeness," from immaturus "unripe, untimely" (see immature). Meaning "lack of maturity" attested from c. 1600.
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sophomoric (adj.)
"characteristic of a sophomore" (regarded as self-assured and opinionated but crude and immature), 1806, from sophomore + -ic.
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half-baked (adj.)
1620s as "underdone;" colloquial figurative sense of "silly, immature" is from 1855; see half + bake (v.).
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imperfect (adj.)
late 14c., imparfit, "sinful, immoral; not properly formed, not complete, immature; rudimentary, elementary," from Old French imparfait, from Latin imperfectus "unfinished, incomplete, immature," from assimilated form of in- "not, opposite of" (see in- (1)) + perfectus "finished, complete" (see perfect (adj.)). Altered mid-16c. to conform to Latin, along with perfect. Related: Imperfectly.
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unfledged (adj.)
c. 1600, of persons, "immature, not experienced," from un- (1) "not" + past participle of fledge (v.). Literal sense of "not yet covered in feathers" is recorded from 1610s.
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puerile (adj.)

1660s, "youthful, boyish," a back-formation from puerility (q.v.), or else from French puéril (15c.), from Latin puerilis "boyish; childish," from puer "boy, child." The depreciative sense of "merely juvenile, immature, lacking intellectual force" is from 1680s.

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childish (adj.)

Old English cildisc "proper to a child;" see child + -ish. Meaning "puerile, immature, like a child" in a bad sense is from early 15c. Similar formation in Old Saxon kindisc, Middle Dutch kintsch, Dutch kindsch, German kindisch. Related: Childishly; childishness.

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juvenescent (adj.)

1759, "becoming young, growing young in appearance," from Latin iuvenescentem (nominative iuvenescens), present participle of iuvenescere "to grow into youth, grow young again, regain youth," from iuvenis "young man" (see young (adj.)). Its use in a sense of "immature, undeveloped" (by 1875) is etymologically incorrect.

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unripe (adj.)
Old English unripe "premature" (in reference to death), from un- (1) "not" + ripe (adj.). Meaning "immature, not fully developed (of fruit, etc.) is recorded from mid-13c. Similar formation in Middle Dutch onrijp, Old High German unrifi, German unreif.
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