It's a solidus / schilling from King John Casimir II of Poland-Lithuania. Yours has the horse-and-rider of the Lithuanian half of the country (MAG.DUC.LIT = Grand Duchy of Lithuania). The coin would bear a date somewhere the 1660s, I believe, but unfortunately that part of your coin is either worn away or off the flan.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis