frugalyoudaoicibaDictYouDict[frugal 词源字典]
frugal: [16] Paradoxically, frugal comes from a source that meant ‘fruitful’. English borrowed it from Latin frūgālis, which was derived from the adjective frūgī ‘useful’. This in turn was the dative case of the noun frūx ‘fruit, value’, which came from the same base as frūctus, the source of English fruit. The links in the semantic chain seem to have been that something that was ‘useful, valuable, or productive’ was also ‘profitable’, and that in order to be ‘profitable’ it must be ‘economical’ – hence frugal’s connotations of ‘careful expenditure’.
=> fruit[frugal etymology, frugal origin, 英语词源]
gayyoudaoicibaDictYouDict
gay: [13] English borrowed gay from Old French gai, an adjective of uncertain origin connected by some with Old High German gāhi ‘sudden, impulsive’. ‘Happy’ is its ancestral meaning, stretching back to Old French gai. The 20thcentury sense ‘homosexual’, which first came into general use in the 1950s, can probably be traced back to the 17th-century meaning ‘sexually dissolute’.

By the early 19th century it was being applied specifically to the world of prostitution, and it seems not unlikely that male prostitutes and their male clients could have been the vector for the present-day usage. A reported 1868 song by the US female impersonator Will S. Hays was supposedly called ‘The Gay Young Clerk in the Dry Goods Store’, but it is not entirely clear what ‘gay’ is supposed to have meant here, and the earliest reliable printed record of the ‘homosexual’ sense is from 1933.

The adjective underwent a further semantic flipflop in the early 21st century, when kids’ slang commandeered it, paradoxically, for ‘sad’.
recluseyoudaoicibaDictYouDict
recluse: [13] A recluse is etymologically a person who is ‘shut up’. The word was borrowed from reclus, the past participle of Old French reclure ‘shut up’. This was descended from Latin reclūdere, a compound verb formed from the prefix re- ‘again’ and claudere ‘shut’ (source of English close) which originally, paradoxically, meant ‘open’ – the notion being ‘reversing the process of closing’. ‘Shut up’ emerged in the post-classical period.
=> close
paradoxical (adj.)youdaoicibaDictYouDict
1580s, from paradox + -ical. Competing forms were paradoxal (1560s), paradoxial (1620s), but they survive in niches, if at all. Related: Paradoxically.