Have One's Wits About One Meaning

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.