When you are having an extra marital affair,or bumping uglies with someone other than the person you are married to.
When you're having sex with someone other than your spouse and usually one of the spouse's is unaware that his/her mate is having sex outside of their relationship.
That old Tiger... He can't keep his putter in his pants. I wonder if his wife knows He's such a Cheetah'?
Tiger sure bangs all the babes! He's is such aCheetah'!