Something good happens.
A state of happiness.
For example you're late for college, you think you're going to miss the bus but you get it in time. Poptastic!
You open your bank statement and find that you have money in there- poptastic!
Your favourite chocolate bar is going in the shops for half price, so you can buy two for the same price... Poptastic!
When someone asks how you are feeling you reply Poptastic!
When something wrong happens, or something bad.
For example, you buy your favourite drink only to find that you bought the wrong flavour... unpoptastic :(
When life gets you down its unpoptastic :(