Calm, gentle, untroubled evenness.
Calm, gentle, untroubled evenness — an unexcitable temperament, sometimes shading into blandness.
Calm, often unexcitable evenness — a steadiness of temperament or surface not easily disturbed (a placid lake). Its distinguishing edge is a frequent connotation of mildness and, at the negative pole, blandness, passivity, or complacency — it can suggest a placid contentment that verges on dull or unmotivated. Less dignified than serenity or equanimity (which connote earned composure) and less active than composure (self-possession under pressure).
1610s, from Latin placiditas, from placidus “peaceful, gentle, calm,” from placere “to please” (the same root as please).
Entered English in the early 17th century carrying its Latin senses (“peaceful, gentle, calm”), and the meaning has been stable; the faintly pejorative shade (passivity, complacency) is a modern usage nuance rather than a dated sense change. No reliable recent-generation shift.