An ancient Welsh colloquilism describing a "person" who indulges in an act of cruel and unusual beastiality whereby an onion is used to sodomize ruminant livestock. Specifically where a leek is used to penetrate the rectum of a sheep for perverse sexual gratification.
From the noun; 'to welp'
Daffyd looked at Holly with a mixture of confusion and bewilderment upon his simple innocent face. "But I don't understand Holly, what do you need that leek for??"
"Shut up and hold that bastard sheep still isn't it!!" Snapped Holly fiercely, her face set in grim determination as she brandished the leek menacingly.
Afterwards, Holly held the dirty leek aloft in the air, with a manical grin she waved it around like a sordid welping trophy as Daffyd sat sobbing on the grass, traumatized by what he had just seen. He tried in vain to comfort the sexually injured sheep. Eventually Holly sat down beside him, looking both calm and spent but also deeply satisfied. "Yes Daffyd" she said, with what sounded like heavy solemness in her voice "I am a welper."