The term is old French and comes from the time of the first crusades - most of the crusaders came from French-speaking areas.
Crusaders needed money to finance their expeditions. They often mortgaged their lands to wealthy religious institutions and nobles.
If they survived the crusade, they would pay back the money. If they’d die, the noble or institution received their collateralized land.
Mort = death
Gage = pledge
In old French. And the term is still used today. Sinister meaning and story behind it though.