IPA: /kəmˈpleɪ.sən.si/
KK: /kəmˈpleɪn.sɪ/
A feeling of being satisfied with oneself or one's situation, often without realizing potential problems or dangers.
His complacency about his job performance led to serious mistakes.
Complacency is formed from "com-" (meaning together or with) and "placere" (meaning to please). The word describes a state of being pleased with oneself, often to the point of self-satisfaction or unawareness of potential dangers.
Think of being 'pleased' ('placere') with yourself 'together' ('com-') — that's why complacency means being self-satisfied.