This has moved to #1 on my list of Phrases that Should Be Banned.

It means the opposite of allrighty! It means "I heard what stupid things you said and while it'd be silly for me to openly disagree, I'll simply through this out there so everyone knows how vehemently I disagree with you and think you are an idiot". Worse, it's almost always used in retort to some opinion that actually makes decent sense.

Moved to #2 is "Whatever". This actually means "I am not sentient enough to formulate and provide any cogent comments".

Feel free to add your own.