The present invention is directed to a lock construction for securing to a trailer socket and thereby preventing attachment of the trailer socket to a connecting member of an unauthorized towing vehicle. The lock construction comprising a base with a first upstanding member extending from the base and adapted to engage the trailer socket of the trailer. A second upstanding member is disposed on the base in spaced relation to the first upstanding member. A shaft like member is positioned in the housing and is slidable toward the first upstanding member from an unlocked position where the third member is spaced away from the first upstanding member to a locked position wherein the third member engages the trailer socket once it is attached to the first upstanding member. A locking mechanism is operative connected to the third member and to allow slidable movement of the third member. A ratchet member and a ratchet surface are also included to allow slidable movement of the third member to different locked locations.