Frum is a Yiddish word. It is approximately equivalent to "religious" or devout. In common usage, it is used to indicate that someone is an Orthodox Jew. While technically an adjective (e.g.
Dost ist a frumme Yid -- That is a religious Jew), it is often used in Yinglish as a noun. Or rather, the diminiutive is. Mostly in a poking-fun-at-self manner.
More here, although he's wrong about it not also being used self-referentially, at least in some circles.
Correct usage of frum and variants thereof:
He went to Israel last year, and totally frummed out!
I'm looking for a doctor, preferably a frum one.
She is such a frummie! So holier-than-thou, she makes me sick.
How long have you been frum?
And of course, the age-old question, asked by match-makers everywhere: Are you a BT (ba'al(as) teshuva, one who became frum as an adult) or an FFB (frum from birth)?
Better?