www.alphadictionary.com

We all know what "eight" means: a cardinal number that is one more than seven or two less than ten. The mystery to me is how it got from the simple Old English ehta, eahta to the convoluted spelling we have today. Latin and Greek preserved the Indo-European root of the word; why didn't English? –– Mike Nichols

Blame it on the scribes, or so I gather. They decided to go for this combination of letters to imitate what at the time was a hard H. Then it just solidified as standard spelling, even when the hardness went soft and nearly disappeared.

A bigger mystery to me is the French term for ten times eight. Eight = huit, but eighty = quatre vingt. Not a nice simple word; you have to do arithmetic.

That's because they used a vigesimal numbering system way back when.

I found a breezy little article to explain it a bit.

This post from the Agora might pique some interests, too.