Wiktionary
prep.phr. (context idiomatic English) As a profession; professionally.
Usage examples of "by trade".
And while I am sitting there, who comes in but Sam the Gonoph, who is a ticket speculator by trade, and who seems to be looking all around and about.
He is a horse player by trade, and eating is really just a hobby, but he is undoubtedly a wonderful eater even when he is not hungry.
His first name is Jonas, and he is a housepainter by trade, but he seldom feels like doing any painting, as he claims he never really recovers from a terrible backache he gets when he is in the Spanish-American War with the First New York, so Miss Alicia Deering supports him by dealing them off her arm in the Commercial Hotel.
Armand Fibleman is a character who will bet on baseball games from who-laid-the-chunk, and in fact he will bet on anything whatever, because Armand Fibleman is a gambler by trade and has been such since infancy.
Fatso is a horse player by trade and he is en route to Miami to participate in the winter meetings at Tropical Park and Hialeah, and he leaves New York City with just enough money to get him as far as West Palm Beach by bus, but with nothing over for food and drink on the journey.
It seems that after Miss Clarabelle Cobb goes back to her old home in Akron, Ohio, she winds up marrying a young guy by the name of Joseph Hatcher, who is a bookkeeper by trade, and has a pretty good job in Akron, so Miss Clarabelle Cobb and this Joseph Hatcher are as happy as anything together for quite a spell.
I just go on eating and listening and Ambrose seems about ready to burst into tears as he tells me about his suffering because of his love for Hilda, when who comes into the Canary Club all dressed up in white tie and tails but a guy by the name of Brogan Wilmington, who is what is called a playwright by trade, a playwright being a guy who writes plays which are put on the stage for people to see.
Judge Goldfobber, who is a lawyer by trade, sends word to me that he wishes me to call on him at his office in lower Broadway, and while ordinarily I do not care for any part of lawyers, it happens that Judge Goldfobber is a friend of mine, so I go to see him.