Not all there
Definition 1 of 1
Pro Tip 1/3Context Clues
Look for context clues around the phrase to determine if it’s about mental presence or mental health. 
He forgot his keys again; he's not all there today.
Pro Tip 2/3Colloquial Tone
It's used in informal conversations and may not be suitable for formal contexts or writing. 
Ever since she retired, she seems not all there when you chat with her.
Pro Tip 3/3Potential Offense
Be aware it might offend if someone interprets it as a remark on their intelligence or mental health. 
Whispering that he's not all there might upset his family.