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.
As a matter of interest, B, the earliest recorded use of the word in English - dated 1859 - was in a book called 'Artillery Man' by F A Griffiths. My guess is that he himself was an artillery man during the days of the Raj in India, as you suggest.
That Tefler lady is in trouble when she gets up later! She's my partner, Lankeela, and obviously did not sign out after using AnswerBank yesterday evening. Of course, I did not notice, but - believe me - the answer above is mine.