Buckle up Buttercup is a sarcastic expression, denoting that one should get ready for some upcoming unpleasant or extreme action.
It is a bit of a sarcastic portmanteau between the "buckle up" phrase used to get people to use seat belts and "pucker up buttercup" meaning to prepare for a kiss. The sarcasm reverses the positive connotations.
(Sir Butterfluff by day) A super hero/villain on day 2 who only comes out in the snow. he wears a shower curtain cape, has a teaspoon as a weapon, has a crackly old man voice and sometimes combines the macarena with the chicken dance.
girl: "lol is that guy wearing a shower curtain??" Sir butterfluff: "ohh how very dare you! you sillyold girl, for I am captain butterfluff!! oohhh"