underhandedly

This term carries a strong negative connotation, implying a deliberate breach of trust or a violation of fair play. It describes actions that are not merely secret, but intentionally deceptive, often involving a "behind-the-scenes" manipulation to gain an unfair advantage.

In terms of register, it is more descriptive and judgmental than the neutral "secretly" or the formal "surreptitiously." While "surreptitiously" focuses on the act of avoiding notice, "underhandedly" emphasizes the moral dishonesty and the opportunistic nature of the behavior.