In Middle English, used of the weather. Current sense of "easily flexible" is from c. 1300. Related: Litheness. Old and Middle English had the related verb lin "to cease doing (something)," also used of the wind dying down.
late 14c., relenten, Anglo-French relenter, "to melt, soften in substance, dissolve," ultimately from re- in some sense + Latin lentus "slow, viscous, supple" (see lithe), perhaps on model of Old French rallentir, "but the immediate source is not clear" [OED]. Figurative sense of "become less harsh or cruel, soften in temper" is recorded from 1520s; the notion probably is of a hard heart melting with pity. Related: Relented; relenting.
For modern tree names from adjectives, compare aspen. OED suggests the use of the adjective as a noun is at least partly creditable to "translations of a German romance" (German linden is the plural form and the form used in compounds).
"slender, lithe," 1817, from French svelte "slim, slender" (17c.), from Italian svelto "slim, slender," originally "pulled out, lengthened," past participle of svellere "to pluck or root out," from Vulgar Latin *exvellere, from Latin ex- "out" (see ex-) + vellere "to pluck, stretch," from PIE *wel-no-, suffixed form of *uelh- "to strike" (source also of Hittite ualh- "to hit, strike," Greek aliskomai "to be caught").
plant genus, mostly herbaceous climbers, 1550s, "periwinkle," from Latin clematis, from Greek klematis, in Dioscorides as the name of a climbing or trailing plant (OED says probably the periwinkle) with long and lithe branches, diminutive of klema "vine-branch, shoot or twig broken off" (for grafting), from klan "to break" (see clastic).