Continent

From Thikipedia

Continent (adjective): Restrained or controlled, able to show self-control. Also, Having normal voluntary control of excretory functions, not in need of a change of underwear.

The opposite of continent is 'incontinent', or 'incontinence'. Of course, they make special pants for that these days. An individual who displays voluntary control of excretory functions above and beyond the call of duty is occasionally referred to by the slang term 'supercontinent'.