She left her husband and ran off with some young 'stud'.
retired racehorses being used as 'studs'
a horse that has been 'put out to stud'
(..) I bet Russian czars never paid that for a single horse. Khartoum. Khartoum. I'm not going to race him, though. I'm going to put him out to stud. Thanks, Tony. Let's get something to eat. Corleone is Johnny's godfather. To the Italian people that's a very sacred, close relationship. (..)1972 The Godfather