The Santa Claus is a sexual act in which a person is anally fucking their partner, and then they pull out and begin to give a rim job. Their partner, a gassy individual, farts all of the accumulated cum into the face of the rim job giver.
The resulting appearance is a white beard resembling Santa Claus.
Bob and Tom were having butt sex. Tom started to give Bob a rim job, and Bob gave Tom the Santa Claus