MGR = map grid reference, a position on the map.
When I was in army, there was once when my section got lost in the forest. We were on an assessment on our map reading skills. Armed with a signal set, compass, and map, we were to find our way in the forest to the various checkpoints. Unfortunately, we seemed unable to find one of the checkpoints and we could not make out where we were in the forest. The conclusion was we were lost, so we decided to contact the command post for help.
“What is your MGR?” came the response from our instructor in the command post.
What a strange question. If we knew our location, we would not be lost. We replied that we did not know, but the instructor insisted on our MGR. It was very vexing. Sadly we could not end the conversation as there was protocol to follow: only the higher ranking person can end the conversation. So in a bid to end the conversation, we gave a rough estimation of our location based on the last checkpoint we passed. The instructor gave some directions and ended the conversation.
Thankfully we eventually found our way out.