The Melty Man is a Jeffism.

According to Jeff, "He" is the "arch-enemy of trouser confidence", causing men to lose their erections when they are in bed with a woman, by making them think too much about what they are doing and if they are doing it right. Jeff describes him in The Melty Man Cometh as "The arch enemy of trouser confidence, Professor Moriarty in groin form, Darth Vader...without the helmet."

According to Jeff and Steve (who, uniquely, agrees with Jeff about this character), you can't say his name, or even think about him, without him coming to "do his evil work inside your terrified pants".