A human sock is a person (usually female) who's only redeeming quality is being proficient at sexual intercourse, or is very willing to participate. Can also be used for a Nymphomaniac. Refers to the act of masturbating into a sock to avoid stains on upholstery/textiles.
"What about Sarah, is she DTF
"Well yes, but don't ask anything else of her. She's a human sock.