A court martial is a military court that tries soldiers. Face a court martial.
The adjective follows the noun, which is why the plural is courts martial. A court martial applies military law, which covers offences no civilian court knows.
1nouncourt martialA military court that tries soldiers.
“The court martial lasted four days and cleared him of everything.”
court martial is a court.
From French cour martiale, which is why the adjective follows the noun in English.
French
cour martiale
English, today
court martial
A French cour martiale, hence the unusual order of an adjective following the noun.
Etymology from Wiktionary, CC BY-SA