(British, law-enforcement) If a police officer is said to be wearing "half-blues", s/he is partly wearing his/her uniform (blues) out in public, but not in their full kit. Usually, this is done when an officer is travelling from/to their place of work. Although this primarily for police officers, the term can be used for other professions where a person is not in full uniform.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.