The word ambush as a noun was first used in 1489, but it’s been around as a verb since around 1300, when it was first used in a biography of Thomas Becket, a canonized archbishop of Canterbury. Back then, it was spelled abussede, and people were clearly confused about how to properly spell the word, because it also took on forms like enbuschyt, enbussh, embusshed, embushe, and inbush, with its current spelling only becoming normal in the seventeenth century. It all comes from the Old French verb enbuscier. This had the same definition as today, but more literally meant "in forest", because the prefix en- meant "in" and the root is Latin boscus, meaning "wood". The idea is that a good way of ambushing someone is hiding in a nearby forest. En-, through Latin in, and under influence of Frankish an, derives from Proto-Indo-European hen, also meaning "in". Boscus traces to Proto-Germanic buskaz, which meant "bush" (and is indeed the etymon of English bush), and that's from PIE buh, "to grow".
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.