This under-utilized phrase is an exclamitory reaction to a surprising tidbit of
news. Similar to the phrase "Tits in my ass," "Dicks in my dick" is essentially upping the ante to improbable heights, thus describing the utter shock, disbelief, or outright joy that the one uttering this phrase
may have concerning the current situation. While this phrase
may sound innately homoerotic, the phrase essentially stems from the improbability of actually having a penis lodged within another, thus creating the initial surprise.
Person 1: "Tits in my ass, I just
scored awesome seats to the Slayer concert!"
Person 2: "That's sick
man, did you manage to get one for me too?"
Person 1: "Hell yeah
man, of course."
Person 2: "Dicks in my dick! I can't wait!"