n. A person who is involuntarily celibate; the state or condition of being involuntarily celibate.
Other Forms
One man has a suggestion to solve socially-awkward men's dating woes: Institute a government program which would pay women to go on dates with them.

This is evidently not a joke. The blog "governmentsgetgirlfriends" is dedicated to discussing the issues of "incel" men (short for "involuntary celibacy").
It can be argued that incel is a symptom of a deeper root cause and that this cause should be the greater focus of investigation. The most obvious reasons for some incels would be social phobia or a significant degree of social incompetence.
—“About,” The Incel Project, August 20, 2008
2002 (earliest)
I've lost track of the number of men's groups, lists, and boards that I have been part of - I know that it is well over 100 - and it is very rare that they grow over about 50 active members before they start tearing themselves apart with internal conflict. We've just had an example of it here with the sudden introduction of the "incel" concept which in short order led to a suggestion to split the board.
—“Zenpriest #58 - The MRM's Perpetual War With Itself,” No Ma'am, February 27, 2002