at wit's end
 at (your) wit's end  (chiefly US)   ( or   at (your) wits' end)
  : not able to continue thinking or trying to solve a problem : upset and unable to think of what needs to be done
   I've spent six hours trying to fix my computer, but now I'm at my wit's end.
   She's at her wit's end trying to keep her brother out of trouble.
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 Main Entry: wit