1. once in a blue moon - Adverb
2. once in a blue moon - Phrase
once in a blue moon (not comparable)
(idiomatic) Very rarely; very infrequently.
Synonyms: see Thesaurus:rarely
Once in a blue moon someone like you comes along. Van Morrison
Magic doesn't happen often - not once in a blue moon ... I expect there isn't another magic ship like this one in the whole world. Hilda Lewis
Another thing that I don't like to do is show too much how it goes. I do it once in a blue moon. Sometimes there are lessons when I don't pick up a violin at all. Itzhak Perlman
Once in a blue moon a captain imposes his influence not just upon his opponents, but on a generation of cricketers, by the sheer force of his personality. James Coldham
Because of my new bangs I’ve gone from blow drying my hair once in a blue moon to blow drying my hair every single day. Source: Internet
How I craved the after-dinner smoke with a cup of coffee (and once in a blue moon still think about)! Source: Internet