WordNet (r) 3.0 (2006):

add-in
    n 1: a printed circuit that can be inserted into expansion slots
         in a computer to increase the computer's capabilities [syn:
         circuit board, circuit card, board, card, plug-
         in, add-in]

Hitchcock's Bible Names Dictionary (late 1800's):

Addin, adorned; delicious; voluptuous