Context

This line is written by the author of Job in Job 40:15.

In case you aren't familiar with Job's story, he had a pretty hard life. And at this point, he's been complaining for a little while, so God finally interrupts him to remind him, "Hey—I'm in charge here. Everything's under control. Exhibit A? Check out behemoth, which I made." (Which actually might be a dinosaur.)

Where you've heard it

It's hard to say exactly what Job was talking about here, but today a behemoth is pretty much any really big thing—a St. Bernard, a Hummer, Andre the Giant, etc. (See also, Leviathan.)

Pretentious Factor

If you were to drop this quote at a dinner party, would you get an in-unison "awww" or would everyone roll their eyes and never invite you back? Here it is, on a scale of 1-10.

Pretentious? No, but that's not to say you should go throwing this one around. Behemoth isn't a really well-known word anymore. It might get you some looks.