know one's onions

Raymond ChandlerのThe Big Sleep を読んでいます。
“Somebody’s counting on that. It’s the easiest way to fool them. That or the police. Geiger can collect on these notes, unless you can show fraud. Instead of that he makes you a present of them and admits they are gambling debts, which gives you a defense, even if he had kept the notes. If he’s a crook, he knows his onions, and if he’s an honest man doing a little loan business on the side, he ought to have his money. Who was this Joe Brody you paid the five thousand dollars to?”
"knows his onions" はイデオムでしょう。辞書を見ます。
・Oxford English Dictionary: Be very knowledgeable about something.: And you need to know your onions when you tackle Beethoven, even before you get to the details of the music.
・Collins Dictionary: to be fully acquainted with a subject
・English slang and colloquialisms used in the United Kingdom: Knowledgeable and competent in one's task. E.g."I'll say one thing for Craig, he knows his onions when it comes to repairing car engines."
語源が気になりますね。長くなるので引用しませんが、興味のある方は次のサイト（World Wide Words at http://www.worldwidewords.org/qa/qa-kno1.htm）の記事を読んで下さい。