To defacate, poop, or "take a dump". i.e. relates to "a number 2" or "a deuce"
The expression is derived from the "10-codes" often used by citizen-band (CB) radio operators and emergency professionals to shorten radio transmissions.
The expression was coined from a line in the classic CB-radio chatter movie "Smokey and the Bandit", where Sally Field's character asks Burt Reynolds about a cop "taking a leak on the side of the road." Reynolds tells her that is a "10-100", to which she replies, "I guess that's better than a 10-200."