1610s, "remain at rest" (a sense now obsolete); 1650s as "agree tacitly, concur," from French acquiescer "to yield or agree to; be at rest," (14c.), from Latin acquiescere/adquiescere "become quiet, remain at rest, rest, repose," thus "be satisfied with, be content," from ad "to" (see ad-) + quiescere "become quiet," from quies (genitive quietis) "rest, quiet" (from PIE root *kweie- "to rest, be quiet"). Related: Acquiesced; acquiescing.