Abstract: An improved gladhand security lock apparatus is locked to a gladhand brake line coupler to immobilize a trailer. The body of the security lock apparatus is formed with an abutment surface from which depends a clevis wall and a retainer turned back from the clevis wall and spaced from the abutment surface forming a tongue receiving bight. A guard plate is formed on an end of the body and a clip with an upturned rail member is formed parallel to the guard plate and spaced therefrom. The security lock apparatus is attached to the gladhand coupler so that a tongue of the gladhand is received in the tongue receiving bight, the abutment surface of the security lock apparatus overlies a port of the gladhand coupler, and a retainer flange formed on the gladhand coupler is received between the guard plate and the clip. The security lock apparatus body comprises a barrel. A lock housing is formed in a bore in the barrel perpendicular to and confronting a keeper bolt positioned in a keeper bolt receiving bore.