not (oneself)

not

An interjection negating the statement one has just made and thus rendering it sarcastic. Oh, yeah, I love getting a ton of homework on my birthday weekend. Not!

not (oneself)

Not feeling as one normally should, either physically, mentally, emotionally, etc. I'm sorry for getting upset at you earlier, I'm just not myself today. I know Mary hasn't been herself ever since she lost her job.
See also: not

not oneself

Not feeling physically or mentally well, as in I think there's something wrong; he's not himself, or She seemed to be improving last week, but she's just not feeling herself today. Also see feel like oneself.
See also: not

not

half/so bad Informal
Reasonably good.