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brat [brat 词源字典]: brat: [16] The origins of brat are not altogether clear, but it has plausibly been connected with the English dialect brat ‘makeshift or ragged garment’, as being the sort of apparel a rough or ill-mannered child might wear. This brat first appeared in late Old English as bratt, meaning ‘cloak’, a borrowing from Old Irish bratt ‘covering, mantle’.
[brat etymology, brat origin, 英语词源]
ignorant (adj.): late 14c., from Old French ignorant (14c.), from Latin ignorantia, from ignorantem (nominative ignorans), present participle of ignorare "not to know, to be unacquainted; mistake, misunderstand; take no notice of, pay no attention to," from assimilated form of in- "not, opposite of" (see in- (1)) + Old Latin gnarus "aware, acquainted with" (cognates: Classical Latin noscere "to know," notus "known"), from Proto-Latin suffixed form *gno-ro-, related to gnoscere "to know" (see know).

Form influenced by Latin ignotus "unknown." Also see uncouth. Colloquial sense of "ill-mannered" first attested 1886. As a noun meaning "ignorant person" from mid-15c.
impolite (adj.): 1610s, "unrefined, rough," from Latin impolitus "unpolished, rough, unrefined," from assimilated form of in- "not, opposite of" (see in- (1)) + politus "polished" (see polite). Sense of "discourteous, ill-mannered" is from 1739. Related: Impolitely; impoliteness.
rude (adj.): late 13c., "coarse, rough" (of surfaces), from Old French ruide (13c.) or directly from Latin rudis "rough, crude, unlearned," perhaps related to rudus "rubble." Sense of "ill-mannered, uncultured; uneducated, uncultured" is from mid-14c. Rude boy (also rudie, for short) in Jamaican slang is attested from 1967. Figurative phrase rude awakening is attested from 1895.