My dictionary (Chambers) gives gives "backdoor" as an adjective meanin "Unworthily secret" or "Clandestine".

I usually watch Pointless at tea time and they often have a round where contestants have to think of a single word meeting a certain condition (eg, ending in ious). I am often uncertain (with my own ideas as well as answers by contestants) as to what is one word, what is two words and what is hyphenated. I've also been proved wrong a few times when I've felt sure an answer is/isn't a single word.