To treat someone in a way that shows a lack of respect, or to lower someone's dignity or social position.
From 'de-' (down) plus 'mean' from Old English 'mæne' meaning to intend or behave. Originally it meant to conduct oneself, but the 'de-' prefix shifted it toward lowering status.
The word literally means 'to pull down in rank'—it came from the Old English idea that your behavior showed your social status, so bad behavior literally lowered you!
Complete word intelligence in one call. Free tier — 50 lookups/day.