The names for numbers are perhaps the oldest words in our vocabulary. They are so old that the reasons for their use have been lost. There is an attempt to relate three to beyond through the prefixes tri- and trans-. There is also an attempt to relate five to hand, more apparent in the Polynesian languages than in the Indo-European.
Word Ways: Vol. 20
, Article 13.
Available at: http://digitalcommons.butler.edu/wordways/vol20/iss4/13