broadside

The waves are hitting across the side of the boat.

The dictionary writes "collide with full side with", something like that. I just can't figure out what it is saying.

Broad side is the side of an object that takes up the most area.
In the case of the kayak, the waves were coming up against the side, putting it in danger of capsizing.

There's the expression "he couldn't hit the broadside of a barn" technically meaning he can't throw a ball very well.
since a barn is such a big building, you'd have to be a VERY bad pitcher not to be able to hit it broadside.

I've also heard it used when someone takes you off guard, like with an unexpected question "he really broadsided me when he asked me how much money I make a year"

Thank you. And another question. "take sb. off guard", What's meaning? To make you alert of what he's talking?

means to surprise them.

For instance - you probably would not expect someone to ask you how much money you make.
Since you're surprised they asked you such a personal thing - you are "broadsided" or "knocked over" that they asked you.

Where are you from?

Thank you. I see.

The broadside of a ship is one of its two long sides, as opposed to its bows (the sharp front end) or the stern (the back end). When on ship turns so as to bring its side parallel with something else (e.g. a quay, or another ship), it turns "broadside-on" to it.

In the days of wooden sailing ships, the guns stationed along the sides or a ship (as distinct from the ones pointing over the bows or the stern) were called its "broadside guns". As there were far more broadside guns than bow or stern guns, the way to do most damage to your enemy was to turn broadside on to him and "fire a broadside" - that is, fire all the guns on that side of the ship. That's the metaphor being used if you say that somebody's question "broadsided" you.

But in your original quotation, translatorcz, the writer just means that the waves are hitting the boat side-on.

-and in answer to your question about "taking s.b. off guard": this is a military metaphor. When a soldier is on guard, he is alert and watchful. If you take (or "catch") him off guard, you ambush him when he is not expecting it and isn't ready to fight back.

Thank you. You are all so kind. And every background of the words and expressions are welcomed. Thank you again.