[tur-shee-uh m kwid; Latin ter-ti-oo m kwid]
something related in some way to two things, but distinct from both; something intermediate between two things.
Origin of tertium quid
1715–25; < Latin, translation of Greek tríton·ti some third thing
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
an unknown or indefinite thing related in some way to two known or definite things, but distinct from boththere is either right or wrong, with no tertium quid
Word Origin for tertium quid
C18: from Late Latin, rendering Greek triton ti some third thing
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
1724, Latin, literally "third something," loan-translation of Greek triton ti (Plato), used in alchemy for "unidentified element present in a combination of two known ones."
Online Etymology Dictionary, © 2010 Douglas Harper