grammar - What is the plural of "sir"?

A security guard wishes to address multiple people respectfully at once. Does he say:

Sorry, sir, but this ID is invalid.

or

Sorry, sirs, but your IDs are invalid.

or something like

Sorry, gentlemen, but your IDs are invalid.

Answer

Sirs is the technically correct plural of "sir."

That said, it is rarely used today. Gentlemen is normally substituted.

Messrs. (pronounced MEZ-erz) is, curiously enough, the technically correct plural of "Mr." (pronounced MIST-er) and also hardly ever used, except in writing.