"Tardy George" was General George Sykes who earned that nickname already while he was still a cadet at the US Military Academy. Another nickname of his was "Slow Trot Sykes".
George Sykes (1822-1880), earned the nicknames "Tardy George" and "Slow Trot" Sykes while at West Point.
Some called George B. Mcclellan (1826-1885) "Tardy George" as well. He was even the subject of a derisive poem, "Tardy George", written by George Henry Boker (1823-1890).
Source "George Sykes", Wikipedia. https://en.wikipedia.org/wiki/George_Sykes
Source: George Henry Sykes, Tardy George. http://users.civilwarpoetry.org/union/officers/mcclellan/george.html