Any container used to holdliquid (such as a glass, mug, or a cup), that could be potentially used to cum in, that has been cummed in, or is frequently cummed in.
Dude, don't leave your mug in the living room, Stefan'll use it as a cum cup
A cum-cup is an 8-ounce plastic/paper cup that a group of guys use when they play "cum-in-a-cup" -- a masturbationgame similar to "cum-on-a-cookie." All the guys jack off and shoot their loads into the cup. The last dude to have a cumgasm has to drink all the other guys' cumshots.
Some of our friends came to our pod and we played cum-in-a-cup -- I lost, so I had to drink to drink all the other cadets cum. They gave me the cum-cup to save for next time!