Buster is a noun meaning "(informal) a man or boy, used as an address; something that breaks or stops". Historically, it is from bust (dialectal variant of burst) + -er; the 'man, fellow' sense is American colloquial, originally meaning a remarkable or boisterous person. The visible construction is part of the etymology: buster is not just an unanalyzed simple word. The semantic history matters here: the earlier use is the starting point, and the modern sense, "(informal) a man or boy, used as an address; something that breaks or stops", is the later or specialized English value. The historical chain is anchored in Old English berstan. Close English family members include bust, and burst.