Why "jackass"? The label of this two-pack from Jamaican Pride (3 oz., $1.50) — bought at a "West Indian, American, French grocery" that seems to be Korean-run — is unilluminating, except for the absence of "corn." The short list of ingredients comprises only "flour, sugar, coconut, ginger, spices."
Further reading offers two etymologies. Like the jackass, said an informant for Frederic G. Cassidy's 1961 study Jamaica Talk, this "famous hard biscuit" is "faithful, long-serving and tough." Another informant, for Cassidy's later Dictionary of Jamaican English, co-edited with R.B. Le Page, added that it is "very thin and hard and crisp and when being eaten sounds like the eating of corn by a donkey."
Upshot: By one account, the jackass is the biscuit; by the other, the jackass is you.
1228 Fulton St. (Bedford-Nostrand Aves.), Bedford-Stuyvesant, Brooklyn