New-age Cockney Rhyming Slang for turd. More often used to mean excrement than the 'offensive' variations of the word, although sometimes used as a mild insult.
I've just deposited a Thora Hird in your Gary Glitter Shitter.
refering to a (pass) mark in British universities. In cockney rhyming slang a 'third' is called a Thora Hird the English film, stage, and TV actress. See also Geoff Hurst, and Desmond
He did no work at all and was lucky to get a Thora.