"Oh, I found it while thrifting, it was only a dollar."
b. A shady character.
c. Someone that appears to have no moral judgment, self-respect, or general hygiene to the point where he/she is a total laughingstock that must be avoided.
She dumped him and went out with two other guys on the same day? Holy sketchball, batman!
The phrase originates from the movie "Billy Madison" when Adam Sandler's character makes an ass of himself while trying to fit in in high school...
Guy: "No I won't make out with you!"