煞有介事

煞有介事

Literally means:

act as if it were the case

Actually means:

pretend to be serious about something

介, like this.

This phrase means acting as if something were really the matter. It implies that one pretends to be a saint or pretends to be serious about something. Derogatory.

介，这么。

煞有介事，装模作样，表现出像真有那么回事一样。含有假正经，不实在，故意做给人家看的意思带有贬义色彩。