When you can deal with an unexpected storm and the caterer not showing up on your wedding day without getting upset, this is an example of when you would be described asunflappable.
Origin of unflappablefrom un- + flap, noun + -able
(comparative more unflappable, superlative most unflappable)
- (remaining composed under pressure): flappable
un- +"Ž flap (“upset, stir, scandal, controversy") +"Ž -able