When a man puts ice cream or a similar frozen treat in his mouth, sucks another man's scrotum and then performs oral sex on him. By chilling the testis, the orgasm is believed to be more pleasurable. Alternatively, a "bro" can chill another "bro's" testicals with a cold Smirnoff Ice bottle before performing oral sex.
The term originated in the Down-Low scene amongst African Americans in Atlanta and is played as a party game. In gay orgies, a bro would approach another bro with a Smirnoff Ice and hold it on his scrotum. After the bro's scrotum is chilled, he drinks the Smirnoff while the other man gets down on one knee and performs oral sex on him. The goal of the game is to finish the drink before ejaculating (icing) in the other man's mouth.
Did you see Marquis and DeAndre playing bros icing bros at the party? I didn't think Marquis would be able to finish that drink before icing all in DeAndre's mouth!