What does "hit me like a two-by-four" mean?

Solution 1:

2×4:

Image courtesy of Wikipedia.

Edit: Wikipedia has since corrected itself that this is indeed a 2×6, but you get the idea.

Oh, and the sentence as a whole means that the observation was a big surprise or a great shock; an aha experience or a eureka moment; an eye-opener.

Solution 2:

Solution 3:

A two by four is a common format for a piece of wood:

A length of sawn wood of cross section 2 inches by 4 inches, most often employed as structural framing lumber (dimension or dimensional lumber).

"It hit me like a two-by-four" means that you've been hit pretty hard.

In this context, the author uses it as a simile to convey his state of shock after reading the analysis he quoted. He felt that it explained so well why Asians were often stereotyped as good in science and math that it stunned him.

He explains this further down the page:

I hadn't known about the 1965 Act, and its existence and the above analysis just instantly explained so much so well and without reference to Asians somehow being magically different

In other words, he was really surprised by the quality of the explanation.

Solution 4:

From Word Reference forums:

"Two-by-four" is a standard size of wood used in the UK building trade.

So it means a piece of wood with sides of two inches and four inches.