When a gentleman caller tries to impress his female counter-part by engaging in behavior that would typically be more understated in normal situations.
Kevin: Look at Matt yelling at the TV like an idiot. He barely even likes baseball.
Ray: I know. He's just show-horsing because his girlfriend is here.
When a fat pet or person has trouble jumping or climbing onto a couch or bed & their pudgy legs kick a lot in desperation, it can be said that that pet or person is "river horsing" as they resemble a swimminghippopotamus.