In western Canada this refers to any piece of lumber. It doesn’t have to be strictly two inches by four inches (Canada does use the imperial system in construction).
“We’re gonna build a treehouse. We just need some two-by-fours and a hammer and some nails.”
Definition: Two 4x4 posts bolted/screwed/glued together, to make a single approximately 4x8 board. Invented by a Canadian contractor working on a very overbuilt deck.
A particularly fat, disgusting person in reference to the schoolyard rhyme of the same name:
Fatty Fatty Two by Four
Can't get through the bathroom door So he did it on the floor Licked it up and asked for more