You know him as the large, rotund, and usually tall (and usually dark) man that approaches you from behind at the club. He will then proceed to "freak on" you and you can't do anything about it due to his massive size.
Often associated with rednecks or hillybillys, this word is actually a mispronunciation. Originally, it was meant to be "brother". However due to accents and slurs, it came out sounding like bubba. This is generally attributed to under-educated, southern caucasions but it's also possible that it is from early african american origin.