Etymologically, the second part of the word derives from the Dutch word for pea-soup "Snert". This thick soup typically has the same consistency (and taste, one might say) as diarrhea. The first letter of the word comes from the word "Spurt", as this is generally the way in which the fecal matter is deposited in the undergarments.
NB: the word "Spert" should not be used to define diarrhea in general, or even for diarrhea that sticks to the hand for any other reason than it clinging there on account of a person having checked if he/she soiled his/her pants by zipping a hand through the butt-crack.
Person 2: Are you actually reaching down your shorts to check it? That’s gross!
Person 1: <Reveals hand covered in diarrhea> No, it is Spert!
Joe: John you're such a fucking spert. You're obsessed with sports!