What abbreviation can be traced back to a 1917 letter to winston churchill?

OMG

Explanation:

Step 1: Find the abbreviation

The answer is actually OMG, this was used when Lord Fisher retired and used OMG in correspondence with the future British Prime Minister. This is exatcly what he wrote on September 9, 1917, "I hear that a new order of Knighthood is on the tapis - O. M. G. (Oh! My God!) - Shower it on the admiralty!"

Answer: OMG