What does "by the skin of your teeth" mean — and why is it funny?

informal

Meaning

Only just; by the narrowest possible margin.

Where it comes from

It comes from the Book of Job in the Bible and has been repeated for centuries despite making no literal sense.

Why it is funny

The humor is that teeth have no skin at all, so the phrase measures a narrow escape in something that does not exist. The margin it describes is therefore impossibly, comically thin.

Used in a sentence

"The train was already moving — I made it by the skin of my teeth."