round the bend
Colloquial British English slang for losing one's sanity.
"She's sending me round the bend!" or "Why'd you throw out a perfectly good steak? Have you gone round the bend?!"
'round the bend
UK. One who is mentally imbalanced, eccentric, crazy, etc. Sometimes expressed as "right 'round the bend." An eighties tune, during the second "British Invasion" of music in the U.S., had the lyric, "You send me right 'round, baby, 'right round, like a record baby."
Did you see him talking to that tree? 'Round the bend, he is.
gone round the bend
British slang for someone who is a wacko, slowly losing their mind
“Donald Trump thinks he won the election, I think he’s gone round the bend”