late 14c., "to make (something) less compact or dense," from Old French relaschier "set free; soften; reduce" (14c.), from Latin relaxare "relax, loosen, open, stretch out, widen again; make loose," from re- "back" (see re-) + laxare "loosen," from laxus "loose" (see lax). Of persons, "to become less formal," from 1837. Meaning "decrease tension" is from early 15c.; intransitive sense of "to become less tense" is recorded from 1935. Related: Relaxed; relaxing.