Sergeant-major

(n.) a cup of strongly brewed, sweetened tea

A sergeant-major is the highest-ranking non-commissioned officer in the armed forces. Though lower in rank than the likes of a colonel or a captain, according to Eric Partridge’s Dictionary of Slang and Unconventional English, that standing nevertheless meant that like all the regular recruits, the sergeant-major “could get tea almost whenever he wanted it.”

And apparently enough of them took that offer up for their rank to become a byword for the standard tipple among NCOs in the military slang of the early 1900s: a cup of sergeant-major is a strongly-brewed and sweetened cup tea (often with the implied additional of a tot of rum poured into it too.)