know one's stuff
Also, know one's onions. Be experienced or knowledgeable in one's field or in the matter at hand. For example, Patrice knows her stuff when it comes to Mexican history, or We need a handyman who knows his onions. The allusion in the variant is unclear. [First half of 1900s]
Words nearby know one's stuff
How to use know one's stuff in a sentence
Added to drinking water at concentrations of around one part per million, fluoride ions stick to dental plaque.
In his view, a writer has only one duty: to be present in his books.
Yet this, in the end, is a book from which one emerges sad, gloomy, disenchanted, at least if we agree to take it seriously.
The best comparison here for an American audience is, well, Internet stuff.
The fear of violence should not determine what one does or does not say.
Practise gliding in the form of inflection, or slide, from one extreme of pitch to another.Expressive Voice Culture|Jessie Eldridge Southwick
He alludes to it as one of their evil customs and used by them to produce insensibility.
There was a rumor that Alessandro and his father had both died; but no one knew anything certainly.Ramona|Helen Hunt Jackson
Truth is a torch, but one of enormous size; so that we slink past it in rather a blinking fashion for fear it should burn us.Pearls of Thought|Maturin M. Ballou
Under the one-sixth they appear as slender, highly refractive fibers with double contour and, often, curled or split ends.A Manual of Clinical Diagnosis|James Campbell Todd