Eric Partridge, in Vol II (Supplement) of his Dictionary of Slang & Unconventional English (1937/1970), states that both the usages he gives of Jollop, "strong liquor, esp whisky", & "a laxative; a purgative", are of Australian origin, and states the laxative one to be a corruption of julep, late C19-20.

This just for info & FWIW, as other lexicographers tend to accuse Partridge of having being the sort of compiler who makes up derivations if not sure. He also liked to find Antipodean origins -- maybe because of having been born in NZ & educated at Univ of Queensland?

Jallopy [his spelling], in the 'decrepit old car' sense, he defines entirely separately: there doesn't seem to be any connection.

~M~