Have One's Wits About One Meaning

(idiomatic) To remain calm, composed, focused, or aware.

Example: 1869, Anthony Trollope, Phineas Finn, ch. 3:
  He was confused, half elated, half disappointed, and had not his wits about him.
1889, Horatio Alger, Luke Walton, ch. 10:
  Luke alone seemed to have his wits about him. He saw that there was not a moment to lose, and, gathering up his strength, dashed to the old lady's assistance.
1908, Upton Sinclair, The Moneychangers, ch. 10:
  And if you think he won't have his wits about him, just you try to fool him on some deal, and see.