Fyah is jamaican slang for quite alot things with marijuana. Like Joint, herbs,being high,...
The native jamaicans took the word from their english speaking colonist by hearing the word 'fire'. In the beginning the word was more often used for 'fire' (ofcourse) and a burning joint. But by cultural changes the word became more 'commercialized'.
ex. 1: Make the fyah burning, when the darkest side is touching and croaning
ex. 2: I puff a spliff to walk fyahfyahfyah
Although it may look to say "fire" in some sort of slurred way, it is in actuality an achronim for "F uck Y ou A ss H ole".
n00b: hey i like u. wuts ur aim??
FYAHman: FYAH n00b!!!!