Dayumm. Slang. Urban speak for "damn," unfortunately popularized some time ago by Martin Lawrence. 

Dayumm! Martin -- what happened to you, man? You look like a high-school science teacher who just bought himself a new XL wardrobe after finally resigning himself to middle-aged gut rot. And why not, I guess: He's resigned himself to working with fucking Donny Osmond and Raven Symone -- just cashing in before they take his SAG card away, I guess. And I'm not entirely sure why, but Symone apparently has some sort of lifetime free pass for her work on "The Cosby Show," making her immune to insult, even if her goddamn right boob, in the picture below, looks like it's about to rebel against the fabric of her gown and take flight.