Pensive
Etymology
From French pensif, from penser, "to think," which descends from Latin pensare, "to weigh or ponder" — a frequentative of pendere, "to weigh." The image is of thought as weighing something in the mind. From that literal weighing came the sense of being lost in serious, often faintly sad reflection.
adjective · /PEN-siv/
Deeply and often wistfully thoughtful.
What it names
The absorbed, faraway look of someone turning something over in their mind, usually with a shade of melancholy.
Example
He fell into a pensive silence, watching the rain and thinking about the offer he hadn’t answered.
Related idea
Contemplative suggests calm, sustained reflection, often serene; pensive usually carries a wistful or slightly sorrowful undertone to the thinking.