It was the number assigned to JFK's PT boat that was cut in half by a Japanese destroyer on August 2, 1943
[DOC] [TLE] Motor Torpedo Boat PT-109 - Wikipedia [PAR] PT-109 was a PT boat (Patrol Torpedo boat) last commanded by Lieutenant, 
junior grade .... Around 2:00 a.m. on 2 August 1943, on a moonless night, 
Kennedy's boat was ... Damage to a propeller slowed the Japanese destroyer's 
trip to her own ... explosive fire after PT-109 had been rammed, cut in half, and 
left burning.