In American usage, "bubba" is a relationship nickname formed from "brother" and given to boys, especially eldest male siblings, to indicate their role in a family. For some boys and men, bubba is used so pervasively that it replaces the given name. The nickname may also be used outside the family by friends as a term of endearment.
The linguist Ian Hancock has described similarities between the African language Krio and Gullah, the creole language of African-Americans in the isolated Sea Islands of South Carolina and points out that the Krio expression bohboh (boy) appears in Gullah as buhbuh, which may account for the bubba of the American South.
Robert Ferguson notes in his book English Surnames that bubba corresponds with the German Bube, a boy. This matches Saxon and Hibernian tradition.
Because of its association with the southern part of the United States, bubba is also often used outside the South as a pejorative to mean a person of low economic status and limited education. Bubba may also be taken to mean one who is a "good ol' boy." In the US Army and Marines, bubba can mean a lay soldier, similar to "grunt", but with connotations of endearment instead of derision (e.g., "Can you make that device easier to work with, 'cus every bubba is gonna have to use it.").
At times, it may be used as a term of endearment (or in an insulting sense) for a person, especially a man, who is either overweight or has a seemingly powerful large body frame.
In gun culture, Bubba is a term used for a person who permanently alters or modifies historic firearms, with no regard for its historical value.
The word exists in other languages and carries similar meanings. Bubba is common in Australia and New Zealand as a noun or pronoun to refer affectionately to a baby.