ar-NOT-ee-AY-nus
Here's a breakdown:
* ar- (pronounced like "are")
* NOT- (pronounced like the word "not")
* ee- (pronounced like the "ee" in "see")
* AY- (pronounced like the "ay" in "say")
* nus (pronounced like the "nus" in "onus")
It's a Latin word, and the pronunciation is based on traditional Latin pronunciation rules.