The Latin word sum means what in English?

Sum means "to be" or "I am".
Used in a sentence: "Cogito, ergo sum" means "I think, therefore I am."

http://ancienthistory.about.com/library/bl/bl_verbs_sum.htm
https://en.wikipedia.org/wiki/Cogito_ergo_sum

"Sum" is the first person singular present indicative of the verb "esse" (to be) and translates as "I am".

The modern version of the quoted Descartes' sentence is "Tesco, ergo sum" - "I shop, therefore I am". Tesco is a UK supermarket chain.

"The Church isn't a business and as such business solutions aren't applicable. We are not competing directly with supermarkets and superstores, amusement arcades and parks; rather we are appealing for the hearts and minds of those who shop there to make sure that their shopping does not become their worship. Just as the Hebrew slaves from Egypt built the Golden Calf as the ultimate act of idolatry in the desert, so millions of British people are prostrating themselves before an altar of materialism. It's a big task in a country where Descartes' famous dictum 'Cogito Ergo Sum' (I think therefore I am) has been replaced with 'Tesco ergo sum' (I shop therefore I Am)."

http://www.independent.co.uk/news/people/profiles/john-sentamu-you-ask-the-questions-428924.html