Mortice is a variant spelling of the word mortise.

A mortise is a slot cut into something, in order for another piece (usually a tenon) to fit inside. Usually done in woodworking to join to pieces of wood. This mortise and tenon joint is stronger than just gluing or nailing two pieces of wood together, due to the mechanical rigidity, and also provides increased surface area for gluing.

See what happens when people don't correct their spelling? You get multiple spellings of the same word, horribly confusing non-native speakers.