Ahoj is a Czech (and Slovak & some Polish) friendly greeting.
It is pronounced Ahoy.
It has been around for yonks, and was/is used by Danube boatmen.
It seems more than likely it is related to ahoy and may have been the source of ahoy when British sailors meet Slavic speakers on their journeys.
oops, referring to http://www.straightdope.com/mailbag/mahoy.html
Welcome, jezzaZOZ. You can move ahead two spaces for providing the link.
The word could, of course, have been adopted by the slavs from the Brits they met.
The English speaking world has a bit of an unfair advantage when it comes to etymology. Some lunatics, starting in the 19th century, undertook a massive project to search out all of the words used in printed English back to the beginning. They are still doing it.
To prove that the word existed in a slavic language and was then adapted by the Brits would be tough. Trying to find printed sources that would predate the English are difficult.
Good luck.