'countertonic' originates from an English compound formed from the prefix 'counter-' (ultimately from Latin 'contra', meaning 'against' or 'opposite') and 'tonic' (from Latin 'tonus' via Greek 'tonos', meaning 'tone').

'tonic' comes from Greek 'tonos' -> Latin 'tonus' -> Middle/Modern English 'tonic'; 'countertonic' was created in Modern English by combining 'counter-' + 'tonic' to express a tone positioned opposite/relative to the tonic.

Initially formed to mean 'opposite or relative to the tonic,' the term has come to be used specifically for the pitch a whole step below the tonic (the subtonic) in music-theoretical contexts.