complacent (adj.)

mid-15c., "pleasure, gratification," especially "self-satisfaction, delight in one's condition" (c. 1500), from Medieval Latin complacentia "satisfaction, pleasure," from Latin complacentem (nominative complacens), present participle of complacere "to be very pleasing," from com-, here perhaps an intensive prefix (see com-), + placere "to please" (see please (v.)). The sense of "disposition to please" (1620s) now goes with complaisance.

1640s, "civil and gracious, desiring to please;" 1670s, "disposed to comply with another's wishes;" from French complaisant "pleasing, obliging, gracious" (16c.), present participle of complaire "acquiesce to please," from Latin complacere "be very pleasing to" (see complacent, its doublet, with which it overlapped in sense until mid-19c.). The French spelling possibly was influenced by Old French plaire "gratify." Related: Complaisantly.

"pleased with oneself, self-satisfied," 1760, back-formation from self-complacency or else from self- + complacent. Related: Self-complacently.