A shameless person does not seem ashamed of bad behaviour, even though other people think they should be. The point is the missing blush: they ask, take, lie, or show off as if nothing were wrong. It is a strong moral word, and in Chinese the everyday hook is often 不要臉 — not just “no shame” as a quiet fact, but a slap. That said, English also uses it more lightly. A shameless plug is when you advertise yourself without pretending to be modest; a shameless copy is an obvious copy, done without embarrassment. In those phrases the speaker is half joking, half accusing. The opposite feeling on a face is shamefaced; the opposite judgement of a deed is not quite shameful — a shameful act is one that deserves shame, and a shameless person is one who will not wear that shame. You can be called shameless for the way you refuse to feel it.