I've always said 'combatative' rather than the 'combative'. Possibly because you have argumentative and competitive but also because, for me, combative just sounds wrong - you have to change the stress pattern of the root (combat) and this obscures (again, for me) its meaning.
There are words I mispronounce left, right and centre but the above is a case of me just plain getting it wrong.
There are words I mispronounce left, right and centre but the above is a case of me just plain getting it wrong.