be at (one's) wits' end

To be in a state of distress because one has no more patience or mental stamina, often after having dealt with some kind of problem or difficult situation. The baby's been crying for hours, and I'm at my wits' end! I'm at my wits' end, so if this train gets delayed too, I'm going to scream. Mrs. Smith must have been at her wits' end because she screamed at Tommy for disrupting class.

at one's wits' end, to be

To be at a total loss, completely perplexed. “Wits” here means mental capacity or ability to think. The term was used by Chaucer (Troilus and Criseyde) and William Langland (Piers Ploughman) in the late fourteenth century and has been a cliché since the eighteenth century.