A Tiddlywonk is a term that originated in Europe, its traditional definition is referring to a small or defunct penis. In more modern times it is referred to a young boy who is discovering his sexual desires, or more specifically his ability to masturbate.
"Caught John Doe in the bathroom the other day, think he mighta' been Tiddlywonkin"
A tiddlywonk is a younger person that you admit is better than you in most ways.
"Look at this tiddlywonk kicking my ass!"