When you're nettled, you're irritated or annoyed. You might be nettled by the terrible traffic on the way to work every morning.
Your favorite teacher might be nettled when very few students hand in their homework, and your mom might tend to get nettled when she's at a very loud restaurant and can't hear what anyone is saying. To be nettled is to be exasperated or peeved, and the word developed in the 1400s as a figurative version of the verb to nettle, "to beat with nettles." Nettles, of course, are a prickly, stinging plant.