Words related to lithe
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).
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.