We sometimes use trade to mean occupation. Here, we are saying that his profession is that of engineer.
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I would say that being an "engineer by trade" is another way of saying that you are "not an engineer by college degree". Rather, an engineer by trade has learned to work as an engineer by learning on-the-job.
For example, a person who has worked at a small company designing and cutting metal parts (i.e. lathes, mills, CNC) for 20 years might refer to themselves as an engineer by trade. T