Definition: a person who is lower in position or rank; a subordinate
Why I Looked It Up
This came up a couple times in a book about World War 1, always in the context of British military officers. I dismissed it as a British-ism, but then found it again in a book about American history.
Jackson subaltern Sam Houston, the first elected president of Texas..
Houston served under General Jackson in the War of 1812.
Added on May 2, 2023
Found this in the spy novel Eurostorm:
“NOW!” roared Salisbury. “Get your bloody ass down there yourself, or I’ll have you broken down to a sub-altern before I throw you in jail for the rest of your life…”
Note the hyphenation, which I hadn’t seen before.