Flutter bum is mid-20th-century slang (popular in the 1950s) used to describe a handsome, attractive man—essentially an older equivalent of calling someone a “heartthrob,” “studmuffin,” or “cutie”. Like much slang from the post-WWII era, it was a compound phrase created by combining two existing colloquialisms:

“Flutter”: A reference to making someone’s heart race, skip a beat, or “flutter” with excitement or romantic anxiety.

“Bum”: In mid-century youth culture, “bum” was frequently used as a playful or casual suffix to label a guy who spent his time indulging in a specific lifestyle or trait (similar to terms like ski bum, beach bum, or pool bum).

When combined, a flutter bum literally translated to a guy whose primary function or reputation was hanging around and making girls’ hearts flutter.

So happy you liked it!

Thanks Austin. We use Wikipedia as a reference.