Tad Lincoln was the youngest of Abraham Lincoln's sons, born April 4, 1853. Though his "real name" was Thomas Lincoln, Lincoln gave him the nickname "Tad" because as a baby, the boy had a rather large head. Tad later grew into his head, but he never outgrew the nickname.
Tad is known to have wreaked havoc in all sorts of places, including the Lincoln-Herndon law office and the White House. (He once hitched his pet goat to a chair and had the goat pull him through a party going on inside the White House!)
Lincoln was amused by actions like this and frequently ignored problems Tad caused, to the exaspiration of many. A story is told of a time that Tad accompanied Lincoln to the telegraph office in Washington. While Lincoln was looking over some dispatches, Tad went into the other room and busied himself by drawing on a very white marble tabletop with some very black ink.
Madison Buell, the telegraph operator, grabbed Tad by the collar and dragged him into the room where Lincoln was reading. Buell was outraged and told the President that Tad had ruined the table. Tad, in his typical honesty, held up his black fingers to show that he had, in fact, been up to some fun. At that, Lincoln lifted Tad up into his arms and said, "Come, Tad; Buell is abusing you."
If you'd like to learn more, you may be interested in the article about Tad that I just posted, as well as podcast episode 3 (link opens podcast page) of Everything Lincoln. Or click this link to play the MP3 file for episode 3, "Tad Lincoln."