We found 5 dictionaries that define the word know ones onions:

General (4 matching dictionaries)

1. know one's onions: Oxford Learner's Dictionaries
2. know one's onions, know one's onions: Collins English Dictionary
3. know one's onions: Wiktionary
4. know one's onions, know ones onions: Dictionary.com

Slang (1 matching dictionary)

5. know one's onions: English slang and colloquialisms used in the United Kingdom

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