(verb) To go over-the-top on a sexual joke. A weeth usually occurs when one trys to add on to a dirty joke, but as a result, makes the joke too crude.
Person 1 - "I wonder where that guy's strong hand
Person 2 - "I heard that guy shoved his strong hand
up the principal's snatch, making his hand smell like rotten cheese"
Person 1 - "Way to totally weeth that joke."