Not just "a piece of wood" but "a very specific piece of wood". It's one of the standard sizes for lumber, and the most commonly used, two inches by four inches profile.
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Veho
Tom Bombadildo
Except they're actually 1.5" x 3.5", but called 2x4 because that's the initial size of the cut before it's planed...and cuz it's easier to say 