also: めんへらー
mentally unstable person; nutjob; lunatic
Internet slang, often derogatory. Refers to someone perceived as emotionally or mentally unstable, especially in online communities. Can be used self-deprecatingly.
あの掲示板にはメンヘラが多い。
That forum is full of nutjobs.
彼女は自分をメンヘラだと言っている。
She calls herself a mental case.
That girl seems a bit unstable, doesn't she.
After her boyfriend dumped her, she became an emotional wreck.
Derived from 'mental health' (メンタルヘルス), shortened and altered to メンヘラ. The exact process is uncertain, but it emerged as internet slang.