The word "煞有介事" is formed by combining "煞" as an intensifier meaning "very", with "有" meaning "to have", and "介事" where "介" means "to concern" and "事" means "matter", creating a literal sense of "having a very concerning matter". This structure produces the idiomatic meaning of acting as if one is seriously engaged in something, with the components emphasizing the exaggerated or pretended seriousness attached to an affair.