Etymology
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Words related to childish

child (n.)

Old English cild "fetus, infant, unborn or newly born person," from Proto-Germanic *kiltham (source also of Gothic kilþei "womb," inkilþo "pregnant;" Danish kuld "children of the same marriage;" Old Swedish kulder "litter;" Old English cildhama "womb," lit. "child-home"); no certain cognates outside Germanic. "App[arently] originally always used in relation to the mother as the 'fruit of the womb'" [Buck]. Also in late Old English, "a youth of gentle birth" (archaic, usually written childe). In 16c.-17c. especially "girl child."

The wider sense "young person before the onset of puberty" developed in late Old English. Phrase with child "pregnant" (late 12c.) retains the original sense. The sense extension from "infant" to "child" also is found in French enfant, Latin infans. Meaning "one's own child; offspring of parents" is from late 12c. (the Old English word was bearn; see bairn). Figurative use from late 14c. Most Indo-European languages use the same word for "a child" and "one's child," though there are exceptions (such as Latin liberi/pueri).

The difficulty with the plural began in Old English, where the nominative plural was at first cild, identical with the singular, then c.975 a plural form cildru (genitive cildra) arose, probably for clarity's sake, only to be re-pluraled late 12c. as children, which is thus a double plural. Middle English plural cildre survives in Lancashire dialect childer and in Childermas.

Child abuse is attested by 1963; child-molester from 1950. Child care is from 1915. Child's play, figurative of something easy, is in Chaucer (late 14c.):

I warne yow wel, it is no childes pley To take a wyf withouten auysement. ["Merchant's Tale"]
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-ish 
adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.

The -ish in verbs (abolish, establish, finish, punish, etc.) is a mere terminal relic from the Old French present participle.
childlike (adj.)

1580s, "proper to a child," from child + like (adj.). Meaning "like a child" in a good sense (distinguished from childish) is from 1738. "Sir Gawain and the Green Knight" (c. 1380) has child-gered "boyish, lighthearted."

Childlike and childish express that which is characteristic of a child, the former applying to that which is worthy of approbation, or at least does not merit disapproval, and the latter usually to that which is not: as, a childlike freedom from guile; a childish petulance. To express that which belongs to the period of childhood, without qualifying it as good or bad, child or childhood is often used in composition .... [Century Dictionary, 1897] 
mannish (adj.)

Old English mennisc, mænnisc "human, human-like, natural to the human species," from Proto-Germanic *manniska- (source also of Old Saxon mannisc, Old High German mennisc, Gothic mannisks), from *manna- (from PIE root *man- (1) "man"). In some cases a new formation from man (n.) + -ish.

Sense of "masculine, characteristic or resembling the males of the human kind" is from late 14c.; also from late 14c. in reference to women seen as masculine. As "characteristic of a grown man" (opposed to childish) from 1520s. Related: Mannishly; mannishness. The Proto-Germanic adjective became, in some languages, a noun meaning "human" (such as German Mensch), and in Old English mannish also was used as a noun "mankind, folk, race, people."

Mannish, not closely matching womanish, applies to that which is somewhat like man, as when a boy gets a mannish voice, and to that in woman which is too much like man to be womanly. [Century Dictionary, 1895]