I found this in another forum (written by Cyrus Chiu there), and I thought it was so interesting I decided to reproduce it here.
(1) Why do the Cantonese refer to one's birthday as 牛一?
(2) Why do the Cantonese (or Chinese) refer to a G.I. (soldier) as 丘八?
(3) Why do the Cantonese refer to a rustic and uncouth guy as 十一哥?
(4) Why do the Cantonese refer to a small boy as 豆丁?
A broad hint for the answers: they are all connected with the formation of Chinese characters.