A feeling of smug satisfaction with oneself or one's achievements, often with the implication of being unaware of danger or problems.
From Latin complacentia, from complacere (to be very pleasing to), from com- (with) + placere (to please). The word originally meant 'courtesy' or 'willingness to please' before shifting to mean self-satisfaction.
The shift from 'complacence' meaning 'courtesy' to meaning 'smug satisfaction' is one of English's most dramatic word transformations—what was once a positive social quality became a vice by the 1700s, showing how historical judgment changed about what behaviors matter most in society.
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