The online Oxford gives two senses for
unmate:
- To cause to be no longer mated; to deprive of a mate.
- To separate or disconnect (what had been connected or joined).
In practice the word seems to be mainly used with the second meaning, in relation to electronic equipment, as noted by RTT. An example, from the website of Digi-Key Electronics:
To unmate, a button-style lever on the housing top is pushed down to allow the wire to be disengaged.
Unmate and
unmating will be added as rare words.