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Webster 1913 Edition

Noun.

The state of being pensive; serious thoughtfulness; seriousness.

Hooker.

Noun.

Gloomy thoughtfulness; melancholy; seriousness from depressed spirits.

pensiveness ‎(uncountable)

A thoughtful or reflective state, especially if sad or melancholic.
We left quietly so as not to disturb the widow's pensiveness

thoughtful or reflective state