also: めんへらー
noun
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.