Green’s Dictionary of Slang

1. an epileptic.

1988		J. Healy Grass Arena (1990) 162: Grabs the epileptic by his shoulders and pulls him clear [...] One of the trusties rushes up with pillow kept in the office for just such emergencies. Hunter grabs it, puts it under the e.p.’s head.
2000		N. Griffiths Grits 78: Soon she starst spewin an thrashin about like-a fuckin eppy.

2. an epileptic fit; thus a temper tantrum.

2001		OnLine Dict. of Playground Sl. 🌐 eppy n. derog. [...] 2) An epileptic fit 3) a fit of temper.
2006		D. Mitchell Black Swan Green 294: Mum throws an eppy if I don’t book meals at other kids’ houses weeks in advance.

3. a very stupid or unpopular person.

2001

OnLine Dict. of Playground Sl. 🌐 eppy n. derog. [...] 4) An excessively stupid or unpleasant person.