This happens when you have a severe cold or nasal infection. Your last tissue has disintegrated, your handkerchief is a congealed lump, which should be incinerated to avoid it becoming a hazard to public health, and your nose is blocking up. There is nothing for it but to blow your nose the old-fashioned way. This consists in closing each nostril in turn, with a finger, and snorting vigorously out through the other. The product of this exercise is a snot splatter. Although snot may be a misnomer as the product is usually a mixture of snot, pus and airborne detritus that makes the faecal emissions of a diseased crow’s arse seem positively pleasant in comparison.
It can also be used as a noun to describe the sort of person whose presence makes people instinctively shrink away from them, leave the room as quickly as possible to go through a complete biological decontamination routine.

Someone who used to be a potential girlfriend/boyfriend, but no longer is.