An ancient Welsh colloquilism used to describe a man or woman that engages in the illegal sodomization (under common-law; see 'beastiality') of farmyard ruminants using onions. Specifically, someone who has inserted a leek into a sheep's rectum. From the verb; to Welp.
See; Beastiality, pelping, queebo.
Daffyd looked at Holly with a mixture of confusion and fright on his face, "but I don't understand, what are you going to do with that leek?"
"Be quiet bitch and hold that sheep still!" snapped Holly. With a look of grim determination on her face she began to repeatedly savage the poor ruminant with the leek. Afterwards, as Daffyd sat sobbing on the grass, traumatized at what he had just seen, he tried in vain to comfort the sexually injured sheep. Holly meanwhile, waved the dirty leek in the air triumphantly with a maniacal grin on her flushed face, before sitting down beside Daffyd, with a look of spent calm now on her face. "Yes Daffyd, I am a welper." she confessed tiredly, with what sounded like a heavy note of solemness in her voice.