- fallback
[fallback 词源字典]
- also fall-back, as a noun, "a reserve," 1851, from verbal phrase, from fall (v.) + back (adv.), which is attested in the sense of "retreat" from c. 1600. As an adjective, from 1767 as a type of chair; 1930 as "that may be used in an emergency."[fallback etymology, fallback origin, 英语词源]