john gilpin, n. — Green’s Dictionary of Slang

john gilpin n.

also gilpin

[? tradename, but note the ‘trusty sword’ carried by the eponymous hero of William Cowper’s poem ‘John Gilpin’ (1783)]

(W.I.) a large cutlass with a curved back and flared blade.

1958

cited in Cassidy & LePage Dict. Jam. Eng. (1980).