What is Foenus Nauticum?

Legal Definition
Nautical or maritime interest. An extraordinary rate of interest agreed to be paid for the loan of money on the hazard of a voyage; sometimes called "usura mantma." Dig. 22, 2; Code, 4, 33; 2 Bl. Comm. 458. The extraordinary rate of interest, proportioned to the risk, demanded by a person lending money on a ship, or on "bottomry," as it is termed. The agreement for such a rate of interest is also called "foenu nauticum."' (2 Bl. Comm. 458 ; 2 Steph. Comm. 93 ) Mozley & Whitley.
-- Black's Law Dictionary
Legal Definition
Interest on marine loans.
-- Ballentine's Law Dictionary