Mimochodem

Let me ramble some more about Czech mimo. There's the adverb mimochodem (by the way), which ends in em, a mark of the instrumental case in the singular for neuter and masculine nouns. I'm wondering if the expression wasn't originally mimo chod (out of pace), which was later compressed into mimochod, and then got an extra -em because many adverbs end in -em: like autem (by car), vlakem (by train), letadlem (by airplane). But what intrigues me also is that adverbs of manner are normally derived from adjectives and usually end in ě (e when the preceding consonant can't be used with ě) or less commonly o.