A term either to describe a person (usually female) or a phenomena in which they attempt with a desperation to fit into a pair of skin tight jeans or booty shorts that don't fit at all. The result is people first noticing then running away screaming in disgust. Also, the said jeans or booty shorts look like they are crying for the person to evacuate their space which is beyond any reasonable capacity. Hence why it is referred to as sad pants.
A character generated in the mind of an avid cartoonaphile who is so sad that even the way his pants hang comes across as tragically sorrowful and pathetic.
Oh my God, there’s Sadpants coming toward us, look at his pants and just try your damndest not to startcrying!