Green’s Dictionary of Slang

jossop n.

gravy.

1873		Sl. Dict. 205: Jossop the syrup or juice in a fruit pie or pudding. Also, sauce or gravy. ? School.
1911		J.W. Horsley Memoirs of a ‘Sky Pilot’ 254: Other [words] were new to me, such as ‘jossop’ for broth.
2002		Ham and High 21 June 🌐 This consisted of a liquid, presumably her own invention, that was known as jossop. Made of vinegar, mustard powder and sugar, it was used not so much to dress the salad as to drown it.