a projection at the end of a piece of wood that is shaped to fit into a mortise and form a mortise joint
source: WordNet 3.0
A projecting member left by cutting away the wood around it, and made to insert into a mortise, and in this way secure together the parts of a frame; especially, such a member when it passes entirely through the thickness of the piece in which the mortise is cut, and shows on the other side. Cf. Tooth, Tusk.
source: Webster 1913
To cut or fit for insertion into a mortise, as the end of a piece of timber.