Closet
From dKosopedia
Colloqial term for living or acting in such a way that others do not realize you are gay
Closet first appeared in use as an adjective to describe secret, unknown or private activity in the 1950s most often in connection with drinking and alcoholism. Over the years it became primarily associated with homosexual behavior or more importantly homosexual identity.
The phrase led to the corollary "coming out of the closet" -- first recorded in 1963 -- meaning to publicly acknowledge one's secret identity.
"Coming out" can mean used to mean either coming to a recognition of one's own sexual identity or it can mean in some way publically (perhaps only one-on-one) acknowledging/affirming one's sexual identity.
People usually remain "in the closet" out of fear for themselves, the feelings of others, or their own livelihoods or living situations.
In 2004 it is still legal in 3/4 of the United States to fire, evict or refuse to serve someone simply because of their sexual orientation. It is legal in 47 states to discriminate on the basis of gender identity and expression.
In recent years, the term is often used to describe any hidden activity, sometimes jocularly. A similar broadening of usage covers 'out' and 'outing'. Representative Schmuck outed himself as a closet fiscal liberal.