URUSAI!

Aurelio Asiain writes in the latest issue of Kyoto Journal:

Given the importance of silence in Japanese culture and the attention paid to the voices of insects, I am hardly surprised that in modern Japanese the word urusai - "noisy, annoying, bothersome" - still can be found written with three characters whose literal meaning is "mayfly."

I never knew that. So I looked it up and it's true, although I think most people would probably write a simple hiragana うるさい, and there is another reading 煩い (with a kanji meaning trouble or worry and components suggesting suitably a fire in the head) it can also be written as he said.
五 read "go" means 5
月 read "gatsu" means month
蠅 read "hae" means fly
But put them together and 五月蠅い is read "urusai". Noisy in the sense that someone bugs you by pestering you till you simply cannot stand it. Or noisy in the sense of someone deliberately trying to wind you up. On such occasions a Japanese might let rip with an ill-tempered "URUSAI!" just as they might also swat at an annoying fly.