Anytime a person chooses to have sexual intercourse with another person for the sole purpose of getting that person through an awkward or sad moment. Typically there is no actual interest romantically in that person, but the idea of engaging in intercourse is not horrifying and almost a bit sporty. The giver goes through the motions of actual romance without any actual feelings.
Sally felt bad for Jim during his visit after the loss of his girlfriend
several weeks earlier, and while nothing she tried that afternoon could snap him out of his sadness, she decided to give him a pity-fuck which would show him that happiness could be found again.