Know your onions
Meaning
To be highly knowledgeable or thoroughly competent in a particular subject or area.
Origin
The phrase "know your onions" sprouted in early 20th-century Britain, evolving as a colourful way to describe someone deeply knowledgeable or highly skilled in their craft. While its precise etymology is layered like the vegetable itself, one popular theory suggests it conjures the image of a seasoned greengrocer, who, through years of experience, could expertly distinguish the freshest, highest-quality onions from a mere glance or touch, understanding every detail of their stock. This keen discernment for the subtle nuances of their trade became a metaphor for expertise across any field, from the workshop to the boardroom, signifying a master of their domain.
Examples
- You can always trust Sarah with the accounts; she really knows her onions when it comes to finance.
- The new mechanic clearly knows his onions, diagnosing the problem with my engine in minutes.