Wow, it’s been a very long time since I’ve done one of these. Since… holy crap, last December. Damn.
As a word, eight showed up in the late fourteenth century as eighte, and even earlier as ehte. It’s from the Old English eahta/aehta, which is from the Proto Germanic akhto, and before that the Proto Indo European okto.
As for the symbol… well, that’s where things get crazy. Medieval writing has it as an 8, but in Arabic (where the numerals came to us from) it’s more of a ^ (the opposite of 7, which was that symbol inverted). Earlier, in Hindu, it’s like if it was part of the 8, as was in Brahmi, just with a slightly different part of the symbol. No real explanation for why we switched that digit’s symbol, though. Maybe to distinguish it more? But that’s pure guesswork on my part.
Ah, these posts are so quick and easy. They certainly don’t last nine weeks, that’s for sure.
Tony Jebson’s page on the Origins of Old English