Green’s Dictionary of Slang

(US) diarrhoea or over-frequent need to urinate.

1915		P.W. Long ‘Semi-Secret Abbreviations’ in DN IV:iii 246: b.d.t. Back door trot – explanatory of an insistent ‘call of nature’.
2002		A. Mariello ‘Dirty Dictionaries’ on Weekly Dig 🌐 He suddenly got a serious case of the BDTs and made it to the chapel of ease with the speed of God, only to Hershey-squirt all over the porcelain temple. It ain’t easy being a grasshopper.