Green’s Dictionary of Slang

large, substantial.

1865		Leaves from Diary of Celebrated Burglar 112/2: I’d advise you to take a jackass load in your ‘kicks’ down to Yorkshire, and feed some of your hungry fellows who have come to London for ‘chuck’.
1948		in Randolph & Legman Ozark Folksongs and Folklore (1992) I 428: Thought I heard Lucindy say, / Come on with your jackass root!
1992		Randolph & Legman Ozark Folksongs and Folklore I 428: jackass root, a very large penis (root), since the donkey or jackass has a much larger penis, proportional to its size, than does a horse.