Do or say something dully and you're acting with a total lack of enthusiasm or interest. If you dully tell your host you're having fun at his party, he's not likely to believe you.
A dully droning story will put you right to sleep, and a dully worded speech is unlikely to win you the student council election. In addition to the meaning of "not exciting," dully can also describe things that happen without brightness or clarity, like the stars glimmering dully on a foggy night or a muffled sound echoing dully in a large auditorium. In the 15th century, this adverb was mainly used to mean "stupidly."