The sound eminated from a live fish flopping on the sand as his eyes pop out when you step on him to put a halt to his flap dancing.
Andrew: "Woah Brendan that's a lot of fish you caught!" Brendan: "Yeah nucka.... Hey, one jumped out of the bucket! Grab him Andrew!" Suddenly: BLORT!!! Andrew and Brendan: "Fuckin' Robert, you didn't have to step on him so hard, you twadge!!
Sound effect originally attributed (by myself) to tubgirl.com. Now used as a general word with many applications, such as an expression of disgust or an emphasis of something disgusting or displeasing.
Some Person: Shit I just crapped my pants!!