How is the correct pronuntiation? whatsapp or whasapp?
What´s up. as hey, what´s happening. Or are you talking about the application for your smart phone. What´s App. App as in apple or application.
The person who gave an answer earlier, beware, the word you were looking for was slang, as in street language. never use the word "slag", it means a woman who puts herself about with no moral regard.
When you have two possible native pronunciations for the same word don't ask yourself which one is "correct". Instead, ask yourself in which context/what social group you might find either one or the other ;) In this case the pronunciation without /t/ sound is typically labelled non-standard, thus "slang" or "street".