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.
Words nearby not oneself
How to use not oneself in a sentence
The speaker conjures up centuries of collective sagacity, aligning oneself with an eternal, inarguable good.Politicians Only Love Journalists When They're Dead|Luke O’Neil|January 8, 2015|DAILY BEAST
“Gently rolling hills” roll not-so-gently under my tires, but the English countryside scenery is soporific.
It was promoted on what might be called not-quite-mainstream or, indeed, axe-to-grind media.
In the not-too-distant future, the AMRAAM might also be out-ranged by new weapons that are being developed around the world.Pentagon Worries That Russia Can Now Outshoot U.S. Stealth Jets|Dave Majumdar|December 4, 2014|DAILY BEAST
An autopsy found highly toxic cyanide levels in the blood of the not-so-dearly departed.
Such throats are trying, are they not?In case one catches cold; Ah, yes!
The habit of refreshing oneself with a pipe on some elevated spot which commands a fine view, is common to both sexes.
To suddenly discover oneself proficient where failure had been feared increases self esteem and adds to the sum of happiness.
It is good, I believe, to be respectable, but much nobler to respect oneself and utter the voice of God.The Pocket R.L.S.|Robert Louis Stevenson
Consequently, while the direction of the what-not-how-now line is definitely fixed, their actual positions remain unestablished.