casteyoudaoicibaDictYouDict[caste 词源字典]
caste: [16] Caste has no etymological connection with cast. It is borrowed from Spanish and Portuguese casta ‘race, breed’, a nominal use of the adjective casta ‘pure’, from Latin castus (source of English chaste). The notion underlying the word thus appears to be ‘racial purity’. Use of casta by the Portuguese in India with reference to the Hindu social groupings led to its being adopted in this sense by English in the 17th century.
=> chaste, incest[caste etymology, caste origin, 英语词源]
caste (n.)youdaoicibaDictYouDict
1550s, "a race of men," from Latin castus "chaste," from castus "cut off, separated; pure" (via notion of "cut off" from faults), past participle of carere "to be cut off from" (and related to castration), from PIE *kas-to-, from root *kes- "to cut" (cognates: Latin cassus "empty, void"). Originally spelled cast in English and later often merged with cast (n.) in its secondary sense "sort, kind, style."

Application to Hindu social groups was picked up by English in India 1610s from Portuguese casta "breed, race, caste," earlier casta raça, "unmixed race," from the same Latin word. The current spelling of of the English word is from this reborrowing. Caste system is first recorded 1840.