I've sometimes been reluctant to accept a word not found in any dictionary other than Wiktionary, since it's edited by volunteers and may not always be reliable, and also because it aims to include all words, no matter how obscure. However it does have the great virtue - like its parent publication Wikipedia - of being kept right up to date. The word
incel is an instance where Wiktionary shines.
Its entry carefully defines the various senses of this word, and provides several quotes illustrating its usage. No other dictionary that I checked has the word at all.
The oldest citation given by Wiktionary is from 2001, in an internet news group. But probably the word would have remained right out on the fringes of the language if not for the massacres carried out by men identifying with a misogynistic incel subculture, especially the 2018 murder of 10 people in Toronto by a young man driving a van into a crowd.
I've added
incel as a rare word.