IMAO stands for In My Arrogant Opinion. It is an internet slang similar to LOL (Laugh Out Loud), ROFL (Rolling on Floor Laughing), etc., which is used as an acronym by people during online chatting with others and in email or text messages. It takes less time to type a few characters than a phrase, so it is frequently used among online users and thus gaining popularity in various platforms SMS, emails, messengers, and similar chatting channels.
The expansion clearly suggests the slang is used in a rather unfriendly, rude context. So, the use of IMAO is to be restricted to the informal, day-to-day conversation and not suitable to use in formal settings.
Example: 1
Example: 1Mark: Which shirt do you like? This one or this one?
Peter: IMAO, wear whichever you want. We are late already.
Example: 2
Example: 2Sam: The restaurant is not making profits for the last two years.
Mike: IMAO, sell this restaurant and buy an apartment and rent it for the money.