'Counter' was originally a flat counting table — from Latin 'computare,' merchants tallying sums.
A flat-topped fixture over which business is conducted in a shop or bank; also, a small disc used in board games; or a person or device that counts; or an adverb/prefix meaning 'against, in opposition to.'
From Anglo-Norman French 'countour' (a counting table), from Medieval Latin 'computātōrium' (a place for counting), from Latin 'computāre' (to count, to reckon, to compute), a compound of 'com-' (together) + 'putāre' (to reckon, to think, to prune). The original counter was literally a counting table — a flat surface on which merchants calculated sums using counters (small discs). The prefix/adverb 'counter-' (against) comes from a different source: Latin 'contrā' (against, opposite). Key