relentless adj.

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.

word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.