once in a blue moon
/wʌns ɪn ə bluː muːn/
"once-in-a-blue-moon" is a 15-letter English headword indexed on PlainSpell.
The verdict
“once in a blue moon” is uncommon English (outside the top frequency list), classed as an adverb. The kind of spelling people second-guess.
- Unranked
- below top-frequency English
- 19
- letters
According to Wiktionary data (CC BY-SA, analyzed September 3, 2026) - Very rarely; very infrequently.
Corpus desk
Index EN-once-in-a-blue-moon · once in a blue moon · English
once in a blue moon · unranked · 0 variants · 0 confusables
- FREQ-UNRANKED Unranked
- LEN-MEGA 19 letters
- VOW-8 8 vowels
- CONFUS-0 No pairs
- VAR-0 0 variants
- BOOK-THIN "O" thin
- PHOTO-UNK Peer pending
Read with this headword: Spelling guide for this pattern
Desk codes from Wiktionary headword shape, FrequencyWords rank tier, confusable-pair count, generated misspelling depth, letter-cohort book size, and photo-finish peer. Not a difficulty grade.
| Property | Value |
|---|---|
| Headword | once in a blue moon |
| Language | English |
| Part of speech | Adverb |
| IPA | /wʌns ɪn ə bluː muːn/ |
| Letters | 19 |
| Misspellings tracked | 0 |
| Confusable pairs | 0 |
| Source | Wiktionary (kaikki.org) |
Long-tail rank of “once in a blue moon” in English
Rare enough to double-check
once in a blue moon is uncommon English outside the top frequency list, classed as anadverb, transcribed /wʌns ɪn ə bluː muːn/. Rarity is why readers land here mid-draft. Gloss: "Very rarely; very infrequently.".
The misspelling generator found no plausible variants for once in a blue moon, typically a sign the spelling maps closely to how the word sounds. No close-neighbour confusable shows up for this headword in our dataset, which usually means its spelling is distinct enough that readers don't reach for a similar-looking word instead.
Etymologically, the entry records: See blue moon. First use appears c. 1833. See citation below. The correct English form is once in a blue moon, spelled O-N-C-E- -I-N- -A- -B-L-U-E- -M-O-O-N.
Definition
- 1Very rarely; very infrequently.
Recorded use
Wiktionary examplesThese source excerpts show how once in a blue moon appears in context. They illustrate usage; the definition above remains the entry’s reference meaning.
We are no advocates for the eternal system of producing foreign operas to the exclusion of the works of English composers, but once in a blue moon such a thing may be allowed.
[T]ime expanded and space contracted, and the name Blue Moon took on a symbolic meaning, as if the future, so delicately plausible, were of a kind that might happen once in a blue moon only.
Etymology
See blue moon. First use appears c. 1833. See citation below.
This word in other languages
Definitions, pronunciation, and etymology for this entry are drawn from Wiktionary via the kaikki.org structured extract (CC BY-SA). See the methodology for how each field is sourced and updated.