To dull something means to take away its edge, its energy, or its excitement. "Watching that movie will dull your senses, until you fall asleep."
Let’s think of something interesting about dull. Hmm. It can be a verb or an adjective! So when you’re reading a dull or boring word description, for instance, it can dull or deaden your appetite for reading more word descriptions. That’s pretty fascinating, no? You can even use it for physical things, like a knife, which you can dull, that is, make it less sharp. You can even dull something visual, making it lacking in light, or use it describe pain — you know, the kind that is not sharp, just an ache. Exciting? No — dull.