Lucille is the name B.B. King gave to his guitars. They were usually black Gibson guitars similar to the ES-345-355.
In the winter of 1949, King played at a dance hall in Twist, Arkansas. The hall was heated by a barrel half-filled with burning kerosene set in the middle of the dance floor, a fairly common practice at the time. During a performance, two men began to fight, knocking over the barrel and sending the burning fuel across the floor. The hall burst into flames and was evacuated.
Once outside however, King realized that he had left his guitar inside so he went back into the burning building to retrieve his beloved $30 Gibson guitar. King learned the next day that the two men who started the fire had been fighting over a woman who worked at the hall named Lucille. King did not know Lucille but named that guitar, and every guitar he subsequently owned, Lucille, as a reminder never again to do something as stupid as run into a burning building or fight over a woman.
B.B. King wrote a song called Lucille in which he talks about his guitar and how it got its name. The song was first released on the album Lucille and is included on the B. B. King Anthology 1962–1998 album.