Cajun cooking and Paul Blart movies aside, the boom is a sort of beam that runs along the bottom of a main-sail. It frequently swings across the seating area of the boat -- and will very likely connect with your daydreaming noggin if you're not careful.
Seasoned hands will tell you, "It's called a boom, because that's the sound it will make hitting you."
Seriously? Isn't the arcane and mystifying vocabulary enough as it is, without making up some kind of Cockney rhyming cant to keep newcomers in the dark?
Thus: The first vocabulary lesson of sailing leads directly to the first learning: crouch! When in doubt, get low.
And also, should you enjoy knowing, the word comes to us via the Old Dutch for "wood."
(*Above GIF of Rihanna comes from Giphy.com)