"Femto (symbol f) is a unit prefix in the metric system denoting a factor of 10−15"

> Use “femto-“ in a sentence.

There you go.

You can quote any nonsense in this sentence, so it is an incorrect proof of a word’s existence.

> Use “bfekpv” in a sentence.

Even if we discard obligatory quotes, there's still the hyphen as a part of stand-alone version of this "word". So when I use "femto-" by itself in a sentence, it's at best 6 characters long.