It is the portmanteau from Sloppy and Curry. Slurry means a sloppy curry fart. Its when you follow through when you don't mean to, or when you just put to much pressure behind a fart and you end up shitting your pants.
bloke1: *sloppy fart sound*
bloke2: Tell me that was not a slurry fart
bloke1: I don't think i followed through though
bloke2: go check yourself