The term 'British Arm' refers to the British Armed Forces, which is the unified military forces of the United Kingdom. The two crossword solutions for this term are STEN, a four-letter word, and STENGUN, a seven-letter word. STEN is a family of British submachine guns that were used during World War II. STENGUN is a slang term for the Sten gun, which was widely used by British troops during the war.