Bygone is an adjective and noun meaning "belonging to an earlier time that has passed". Historically, it is a compound of by (in the sense 'past, gone by') + gone, the past participle of go. Both elements are native, from Old English bi and gan. The visible construction is part of the etymology: bygone is not just an unanalyzed simple word. The semantic history matters here: the earlier use is the starting point, and the modern sense, "belonging to an earlier time that has passed", is the later or specialized English value. The historical chain links Old English bi with Old English gan. Close English family members include by, and gone.