at one's wit's end

[at one's wit's end] or [at wits end] {adj. phr.} Having no ideas as to how to meet a difficulty or solve a problem; feeling puzzled after having used up all of your ideas or resources; not knowing what to do; puzzled.

He had approached every friend and acquaintance for help in vain, and now he was at his wit's end.

The designer was at his wit's end: he had tried out wings of many different kinds but none would fly.

Compare: AT A LOSS, END OF ONE'S ROPE.