Backdoor trots

Hi, a friend of mine has just used the saying : 'backdoor trots'. I undertsand what it means but where did the saying come from?

Outhouses or privies are located outside in back of the house, usually.

etymonline says "the trots" for diarrhea is recorded as far back as 1808. It is a play on "the runs". Backdoor (whether this refers to outsides or backsides) adds emphasis (and makes it clear you're not talking about horses running).