The usual use of the word namesake is 'a person named for another', so that the second name-holder, rather than the original name-holder, is the namesake. So usually George W. Bush would be the namesake of George H.W. Bush, but not usually the other way around. Many dictionaries list this as the only sense of the word, and it's probably the one you should stick with.
However, there do exist examples of namesake referring to the original owner of the name rather than the name-borrower. Joseph Addison, writing in The Spectator in 1712, referred to a reader who "Subscribes herself Xantippe, and tells me, that she follows the Example of her Name-sake" (Xantippe was Socrates's wife, an allusion that would have been easily understood at the time).
This would seem to be a recipe for guaranteed confusion, though the problem is not much mentioned in usage guides. Perhaps it just doesn't matter very much.
The word namesake is first recorded in the mid-seventeenth century. It derives from the phrase (one's) name's sake.
I'm assuming Deaver sent it to me because he found a report of someone with the same name as mine. It's one of those nerdy moments that sometimes pops up in an otherwise bland workday.