A term often mistaken for male genitalia. The correct definition of the word is a modern day 'man bag', or the use of a male friend to carry female essentials on a night out/date/general laziness to carry a handbag. Such items include make up and a purse.
Girl: "You're my man bag for tonight";
Guy: "Correction. I am actually your man sack";
Rest of group proceeds to laughter.
Girl 1: "Who is he?";
Girl 2: "I'm not sure, he'd make a good man sack though".