A man who obviously wants everyone he meets to immediately think he has a very large penis. This is because he is very poorly endowed and lacks any shred of self confidence.
A man suffering from Tiny Dick Syndrome (also called TDS) will take his shirt off at the slightest excuse, walk with his biceps flexed at all times, wear sunglasses inside, and brag about how many women he's supposedly slept with at every opportunity. He also drives a very large truck and will gravitate towards anything that will outwardly increase his status while leaving him inwardly an empty husk of a man (with a tiny penis). You will find victims of TDS pretending to have fun at frat parties, having loud conversations with their friends in public, and weeping over their wasted lives when they've had too much to drink. The slightest insult will either shatter their false confidence and leave them a whimpering wreck, or send them into a rage of overcompensation that ends with them challenging everyone in the room to some sort of contest.