None of the online dictionaries I can consult from here give any indication of etymology or first use.

Even more puzzlingly, the word is used in Czech for ‘hello’. I assume they borrowed it, but when and why? It’s a landlocked country.

hope these links help…

http://www.etymonline.com/index.php?search=ahoy&searchmode=none

http://dictionary.reference.com/search?q=ahoy

The sites above give two differing origins, so here’s what SOED says:

1751.  [a int. + HOY.] Naut.  A call used in hailing.

Presumably the interjection “oi!” comes from ahoy.  As for the Czech word, we’ll have to wait for languagehat or someone better qualified than me to tell us.

Ahoy! My name is Paula and I’m new here.

I have read that ahoy was a Viking word. I did a google search and found this link:

http://www.straightdope.com/mailbag/mahoy.html

It might be of some help.

No, no Viking connection here. The Straight Dope has the data for ahoy correct, it is a nautical term and dates to 1751, appearing in Smollett’s Adventures of Peregrine Pickle.

Hoy is an older exclamation, dating to the 14th century. This root, unlike ahoy, is not particularly nautical. The OED2 says it is a “natural exclamation.”

Czech ahoj is indeed from English.  Who knows why the nautical term caught on there?  They may just have liked the sound of it.

Goody! I can put in a plug for John Biggins and his four novels about the Czech officer in the Imperial and Royal Navy Ottokar Prohaska. In one of them Prohaska reconciles the linguistic differences between the Hungarian and German speakers in a Danube gunboat by persuading them to adopt English as the language of command. In passing he also teaches them cricket…

On a gunboat?! they must have got a lot of boundaries!!!