This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. Plural form of mistress.

"Understand that I grew up the daughter of a culture-mad German mother who, when we traveled abroad, would wheedle ballet mistresses from the Kirov into giving my siblings and me private instruction (plying them, if need be, with eclairs and cigarettes), dragged us to Giselle 17 times, and forced us to take piano lessons."