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
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

not

half/so bad Informal
Reasonably good.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
See also: