Tesco � Founder Jack Cohen who, from 1919, sold groceries in the markets of the London East End. He acquired a large shipment of tea from T. E. Stockwell and made new labels by using the first three letters of the supplier's name and the first two letters of his surname forming the word TESCO.
from http://en.wikipedia.org/wiki/List_of_company_n ame_etymologies
I was told it was because his wife was called Tessie Cohen, so he took the first 3 letters of her name and the first 2 of the surname as a tribute to her.