What is a “Bag of Choppy”?

dobegood posted to Reddit:

My mum just used the phrase “we’d be in for a bucket of custard.” I’m not sure if this is a real phrase or she’s having a stroke. Please advise.

My mother once described her new cat, which was very skinny when she got it, as looking “like a bag of choppy”. I asked her what “choppy” was and she admitted that she didn’t actually know – all she knew is that her mother used to describe things that looked like that cat did as looking “like a bag of choppy”. We tried to look it up or work it out but we couldn’t get to the bottom of it – what is “choppy” except for something that comes in a bag and looks like a scrawny cat? – and so we decided to ask her mother, who was then still alive.

My gran was a proper Hartlepudlian, the kind that you could genuinely imagine hanging a monkey, and she was full of old phrases that defied definition. If you ate with your mouth open she’d tell you off for “clacking”, babies were “baens”, and we were always told not to sit on cold doorsteps or else we’d get “chin cough” (I’ve no idea to this day what chin cough is; all I know about it is the mechanism of transmission, and I’m skeptical of that!). Anyway, on our next visit to the North East we resolved to ask my grandmother what exactly was a “bag of choppy”.

Turns out she didn’t know either; it was just a phrase her mother had used.

tl;dr: what is a “bag of choppy”?