It's from Irish Gaelic muise (pronounced /'mʌʃə/) 'indeed'. One etymology I've seen says it comes from mas ea 'if so'. In various places in a sentence, it can become mhuise (pronounced /'wʌʃə/ or /'wɪʃə/), spelled in Hiberno-English as wisha. It also seems to be a common euphemism for Muire (Mhuire) 'the Virgin Mary' (pronounced /'wɪrə/).

Ceci n'est pas un seing.