grammar and crayfish

The Proto-Indo-European root is *gerbʰ- "to scratch". The zero-grade form *gr̥bʰ- became Greek γράφω "to write" and the zero-grade suffixed form *gr̥bʰ-mn̥- became γράμμα "written letter". Γραμματική τέχνη (grammatikē tekhnē) meant "the art of letters" (τέχνη "art, craft, skill" from *teḱs- "to weave, fabricate"), and this was borrowed into Latin as grammatica, which became Old French grammaire "learning, especially Latin and philology". This was borrowed into English as grammar.

The OED tells us that

In the Middle Ages, grammatica and its Rom[ance] forms chiefly meant the knowledge or study of Latin, and were hence often used as synonymous with learning in general, the knowledge peculiar to the learned class. As this was popularly supposed to include magic and astrology, the OF. gramaire was sometimes used as a name for these occult sciences. In these applications it still survives in certain corrupt forms, F. grimoire, Eng. GLAMOUR

So both glamour, which originally meant "magic, enchantment, spell", and grimoire, a manual for summoning demons, used to be the same word as grammar.

There seems to be a similar language/magic association with the word spell.

In Proto-Germanic *gerbʰ- became *kraƀiz-, then Old High German kerbiz "edible crustacean". The "scratch" meaning was extended to crustaceans, because they move by scratching the ground. This was borrowed into Old French as crevice, then borrowed into Middle English as crevise, which was folk etymologized to crayfish and crawfish. Crab is probably from the same Proto-Germanic word.

Some older posts on grammar:

Last year I asked what is grammar, anyway?

The grammar of the Maple Leafs.

What's up with between you and I?

Using that or which in relative clauses.

Conditional clauses: if it was or if it were.

And check out Motivated Grammar's Ten More Common Grammar Myths, Debunked.