1A boy who is relatively old or mature; (also) a boy who is old enough to look after himself or to cope with adult experiences (also (informal) used ironically, of a man).
2 informal A form of address for a man, especially indicating familiarity or approval.
3US informal An important or influential man; (also) a leading figure, organization, etc.; a major player. "to play with the big boys": to compete at the highest level, especially precociously or prematurely.
slang (originally and chiefly US). A piece of heavy artillery.
The term is used of a wide variety of field weapons and projectiles, including armed vehicles.
5A nuclear weapon; an atom bomb or a hydrogen bomb.
Late 16th century; earliest use found in John Higgins (b. c1544), poet and linguist. From big + boy.