An obvious hypothesis is that in olden times, when people went barefoot or
at most wore sandals, they used both their fingers and toes for counting.
This practice survives in modern English only in the insult "you can't count
above ten without taking off your shoes!"
I checked the Oxford English Dictionary, 2nd Edition and found that the first
citation for "threescore" was in 1388, for "fourscore" was in 1250, and for
"sixscore" was in 1300. There were no entries for twoscore, fivescore,
sevenscore, eightscore, or ninescore, which is a little curious. Why would
people only start counting by scores at 60 and quit after 120?
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A similar question: is there any dialects of English that use the expression
"eleventy"? It's not in the Oxford English Dictionary, but I've heard it
often enough not to be surprised by it. Oddly, it is in a way a literary
expression, as it is used by some writers to suggest that a speaker is using
a very unusual dialect. For example, Tolkien opens _The Lord of the Rings_
with a description of Bilbo Baggins's "eleventy-first birthday".
James A Landau