Tell me about the root "empt"

[Northern_Piper]

Actually, here’s the etymology from Etymology On Line:

usher (n.)
c.1380, “servant who has charge of doors and admits people to a chamber, hall, etc.,” from Anglo-Fr. usser (12c.), from O.Fr. ussier, from V.L. ustiarius “doorkeeper,” from L. ostiarius “door-keeper,” from ostium “door, entrance,” related to os “mouth.” Fem. form usherette is attested from 1925. The verb meaning “conduct, escort” is from 1594.

It continues to have a cognate in modern French, “huissier”, which is a court official similar to a court bailiff. There is also the phrase “huis clos” meaning an in camera court hearing (“closed door”).