Complacent
Etymology
From Latin "complacere" (to please greatly), from com- (an intensifier) plus "placere" (to please). When it entered English in the seventeenth century it meant pleasant or obliging — eager to please others. The meaning curdled inward over time, from pleasing others to being pleased with oneself, arriving at today’s note of unearned smugness.
adjective · /kum-PLAY-suhnt/
Smugly satisfied with oneself or one’s situation, and so blind to real dangers or shortcomings.
What it names
A precise label for the quiet failure mode of success — resting so comfortably that you stop noticing what is changing around you.
Example
Two straight years of easy profits had made the firm complacent about competitors it once feared.
Related idea
Contentment is peace with what one has; complacency is that peace curdled into a refusal to notice what has changed.