Stands for Urban Dictionary Sex Syndrome. When someone makes a new entry on Urban Dictionary which is totally innocent, and may or may not be an innuendo. Then some immature 13 year old or a chinless wonder adds a statement to that entry that is completely rude or sexual and has nothing to do with the initial explanation.
UD Sex Syndrome Example: Everyone knows a Slippery Nipple
is an alcoholic shot of Sambuca and Baileys. But I guarentee underneath that is an alternative explanation about someone spunking on a girl's breast. Am I right?