Straw for use in a latrine. The phrase can be read in two ways, offering a nice colloquial play on words. Torche can mean either a 'torch' for the giving of light for which straw could be used, or as a noun meaning 'wipe'. Thus torche-cul means either an 'arse-torch' or an 'arse-wipe'. -
Cf. Longaigne
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.