Rootyeah is a relatively new colloquial adjective emerging from Australia used to show appreciation or admiration. Combining, the word root - a slang term for sexual intercourse and yeah, the word is often used at a time when something is seen as good as a root. Rootyeah is most commonly used as an expression of joy and is best said while gripping the area just above your left wrist with your right hand.
"Bloody hell m8, look at the size of the snapper I just caught!"
"Rootyeah m8, that'll go down well, with my bundy and coke on the balcony this arvo."