wits

The plural gathers a person's mental powers into one working set: keep your wits about you, at your wits' end. In the second phrase the apostrophe follows the s, because the end belongs to all the wits together. Two meanings of wit are normally plural anyway, the "sanity" meaning and the old "senses" meaning. In those, wits is the everyday form rather than a count of anything.

Fright is the usual cause when this meaning appears, and out of is its usual construction: scared, frightened or driven out of one's wits. The phrase pictures shock knocking the mind briefly off its footing, so it suits terror and sudden alarm rather than lasting illness. Like the perceptual use, it exists only in the plural. One loses one's wits, never a wit.

No single wit stands for one sense here. The word works only as a plural, and the singular for "the senses" has fallen out of use. What remains lives in talk of awareness slipping away or coming back after a blow or a faint, as in her wits returned. Here the trouble is perception, not reasoning: the person is stunned, not foolish.