The gage came in two forms: When a feudal tenant or landlord needed liquid resources, they could pledge their estate in land, as collateral, for a money loan.
Since the gaged collateral was typically a piece of real property which generated revenue (e.g. a farm which produced crops, a mill which processed food, a pasture that provided milk or wool, etc.
), the lender-gagee received the rents and profits of the land.
On the other hand, with a dead gage, the property's rents and profits did not go toward satisfying the principal, but instead constituted interest on the loan, which made it a form of usury.
As a result, dead gages were denounced as immoral/illegal among Catholic theologians.