No supporting evidence of Twain saying it either. Also not The Bard
It's a great line. Just not accurately attributed to the great wags of the past few centuries.
In any case my post on this was basically to say
@LSD-25 is an idiot. Which is true, regardless who said it first.
The blood swung its reddest pennant out over the boy’s cheeks, but Trissilian’s mood was not to be resented, or resisted. A battle of wits was to be fought, and the Boy in Blue was unarmed to-night. (Longstreet, 1866)
http://quoteinvestigator.com/2014/06/24/wit-battle/