Refers to a woman in a long-term extramarital relationship with a man who supports her financially, or historically a secondary wife. The reading めかけ is more common in modern Japanese for this meaning.
See also: 妾 (めかけ)
There is a rumor that he keeps a mistress.
"I know nothing of such matters," she said.
concubine; mistress
foreigner's mistress
wife and mistress(es)
illegitimate child; born to a mistress
The kanji 妾 originally depicted a woman under a roof, associated with a secondary wife or concubine. The reading しょう is the on'yomi; the archaic pronoun use is a specialized extension of the character.