A man's sock, usually of the athletic variety, used to catch and collect semen. Boys normally use cumsocks to clean up after ejaculation. Also used...a cumrag. Cumsocks are generally easier to hide as crusty remnants can be easily mistaken for any other substance. Usually used to hide masturbatory behaviors from moms who do the laundry.
I couldn't find my cumsock, so I ejaculated on my chest and cleaned it up with an old t-shirt instead.