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chico (Adjective, Noun) "small;" (masc.) "boy," "child;" (fem.) "girl"
12th cent. Perhaps from Latin ciccus "nothing," earlier "something worthless," but originally "the thin membrane surrounding the grains of a pomegranate." Presumably borrowed from an unattested Ancient Greek word *κίκκος (kíkkos) "shell of a pomegranate," hypothesized by Beekes (2008) on the basis of the Latin word and possible Greek derivatives κίκκαβος (kíkkabos) "small coin in the Underworld," κικκάβι(ο)ν (kikkábi(o)n) "nothing," and κικαῖος (kikaîos), a word of obscure meaning. Ultimately of unknown origin.
Also the origin of the surname Chico.
Indo-European
Romance
Asturian chico, Aragonese chicot, Catalan xic, French chiquet, Italian cica

The sense of "small" was first and then was extended to children. The change from c- to ch- may have been reinforced by Basque, cf. txiki "small," "few," from earlier tiki. Also note 18th cent. colloquialism chicho "small child (who has begun to speak)," from *cic(c)us via double-palatalization distortion found in children's speech (compare niño and ñoño).