A man or boy regarded as the unattractive participant at a social dance; literally, 'the ugly boy at the dance'.
2.
SpainLatin America (widespread)informalpejorativefigurative
A male individual who is treated as undesirable or socially marginal within a group; used figuratively to denote someone overlooked or lacking social appeal.
Examples:
ES: ¿Qué hacemos con el feo del baile?
EN: Hey, Judge, what should we do with the wallflower?
ES: Sr. Cignarelli, por el modo en que está vestido, que no es el feo del baile.
EN: Mr. Cignarelli, from the way you're dressed, that you're no wallflower.