I don’t personally redefine the word. In English, the term “orthodox” means conforming with and accepting what is generally accepted as true.
So a Roman Catholic who is Orthodox would be one who conforms with and practices the faith without picking, choosing and taking exception with the teaching of the faith.
Applied to sin, I would say orthodox people may very well sin…perhaps the very devout Sr.Therese has a problem with gambling away her personal funds to a point it becomes a burden to her order to additionally cover her basic needs, and then lies about how she spent the money. Well, if she is giving in to compulsion but knows and believes she is sinning, I would say she is struggling with sin. It’s not a question of orthodoxy…she accepts church teaching, and let’s say she tries hard to resist, but she is giving into a difficult weakness.
As another matter, say Joe has been taught he should only go to communion in a state of Grace, and should confess regularly to achieve this. He has decided for himself that confession is “dumb” and that he will skip that part of the faith. He also has “no problem” with sex outside of marriage and thinks the church should “stay out of his bedroom”, so has no qualms about living with his girlfriend. This is not only gravely sinful but also unorthodox as he doesn’t even accept the church’s position and teaching that his behaviours are sinful.
So when I speak of an increase in what I see as orthodox behaviors, I am referencing, for example, a noticeable increase in the number of people in line for confession each week. Mass had always been near capacity in the few years I have attended my parish, but I rarely saw people in line for confession. Now I get there when confession starts and commonly, people keep coming the entire set time and I find ten or fifteen people in line the entire time. One priest used to sit in the confessional with nobody to talk to, and now two priests struggle to serve everyone.
There are other expressions of faith whether a woman wears a mantilla to mass, s teen shows interest in studying EF mass, a man takes to wearing a scapular…I don’t know that these are indications inandof themselves of orthodoxy, but unless it’s just for show (which I would not presume of anyone) such choices communicate to me that the individual is seeking greater communion with God and the Church and is seeking means to be more prayerful, so I view them as signs of growth in orthodoxy.