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1833, from French atavisme, attested by 1820s, from Latin atavus "ancestor, forefather," from at- perhaps here meaning "beyond" + avus "grandfather," from PIE *awo- "adult male relative other than the father" (see uncle).
atavism at·a·vism (āt'ə-vĭz'əm)
The appearance of characteristics that are presumed to have been present in some remote ancestor; reversion to an earlier biological type.