wits' end

Noun

 * 1)  The limit of one's sanity or mental capacity; the point of desperation.
 * 2) * c. 1911,, in John Muir and Michael P. Branch, John Muir's Last Journey: South to the Amazon and East to Africa (2002 edition), page 138:
 * Our dozen cabin passengers sorely put to wits' end to pass yesterday without cards in observance of the Sabbath.
 * 1) * c. 1911,, in John Muir and Michael P. Branch, John Muir's Last Journey: South to the Amazon and East to Africa (2002 edition), page 138:
 * Our dozen cabin passengers sorely put to wits' end to pass yesterday without cards in observance of the Sabbath.
 * Our dozen cabin passengers sorely put to wits' end to pass yesterday without cards in observance of the Sabbath.

Usage notes

 * The form wits' end is preferred nearly 3 to 1 in the UK (BNC).
 * The form wit's end is preferred more than 2 to 1 in the US (COCA).