My dictionary, (Concise Oxford, 7th ed.), has it spelt 'numnah'. The origin is given as the Urdu word 'namda' which, in turn derives from the Persian word 'namad' meaning 'carpet'.
Without access to the full OED, I can't cite its first recorded use in English but many Hindi and Urdu words entered the English language during the British occupation of the Indian subcontinent. It's possible, therefore, that this is where the word originally entered into our language.
Chris