姑爷 is a colloquial term used by a wife's family to refer to her husband — essentially 'son-in-law' from the perspective of the bride's parents and relatives. It carries a familiar, in-group tone, so you'd hear it in family settings rather than formal introductions. The term is more common in northern Chinese dialects and older usage; younger speakers may prefer 女婿 (nǚxu) in everyday speech.