John Dennis was a 17th-century English literary critic whose real passion was in writing plays, but he wasn't great at them. One of those plays was Appius and Virginia, a tragedy that required a thunder sound for a rainstorm scene, so Dennis created an instrument for the sound effect out of a bowl and metal balls. The play did terribly and was cancelled after only a few showings, but the theatre manager kept using Dennis' sound technique for a subsequent production of Macbeth. Accounts of the moment vary, but apparently Dennis noticed during a showing that the sound effect was familiar, so he got up and started screaming about how the theatre had "stolen his thunder". After the word got around by help of the local paper, to steal one's thunder entered popular parlance - and that's how we got that phrase!
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.