A person who you dont deem fit to abused by a proper name so therfore you call them a slennis, can also be abreviated to slen.
Can also be used as a light insult to friends and family when there being thick as fuck.
Phillip and Jack are walking and they spot an old man brushing his hair with a shoe coverd in shit. Phillip says to Jack look at the state of that fucking slennis.
James's friend matt is asleep with the hoover in his bed. James find him takes a picture and puts on facebook taggs it and puts a caption look at that slennis.