Uncertain. Possibly from tally (“mate, counterpart”), from Latin talea (“stick, twig”), or from tail, with wag, likely influenced by whack. Compare tallywag, an eighteenth century dialect term meaning either “penis” or “testicle”. Attested as tallywhacker or tallywacker since the twentieth century.
tallywhacker (plural tallywhackers)
- “tallywag; occ. tarriwag”, in Eric Partridge, The Routledge Dictionary of Historical Slang, 2006.
- “tallywhacker”, American Heritage Dictionary of the English Language fifth edition, 2016.