Residual semi-liquefied fecal matter that clings to your palm after you zipped your hand through your butt-crack to check on the cleanliness of your undergarments after letting a fart of doubtful contents rip. The consistency of the fecal matter is key in whether or not the label "Spert" is justified. For instance, if the poop is too hard of substance, then the resulting goo on your hand is not "Spert" but just a shit stain.
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 1: Oh my, I think I may have pooped my pants accidentally.
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!