This is a really interesting post. Before I was a software developer, I worked as a licensed civil engineer. When I think about what is "professional", my former work informs my thoughts.
Certain types of work involve public trust and accountability. As a Civil Engineer, if I was acting "professionally" I was contributing to that trust. If I was acting "unprofessionally" I was deteriorating that trust. Ideally this would mean--if I tell the truth and deliver a safe design that is professional, if I lie to the client, take bribes, deliver a sloppy design that is unprofessional.
But people use shortcuts to make judgements, so the evaluation often comes down to "do you conform to my idea of an engineer?" This others people unnecessarily and is counterproductive.
So when I think "professional" now, I ask myself if the person is acting like their work matters, if they would be proud to put their name on their work, and if what they are doing is ethically sound. But you are right, that is not necessarily what others will hear if I use that word.
I really like using the lens of being reliable and trustworthy. I think you're right that this is what is really at the core of the idea of being "professional".
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