'spar' (noun) originates from Middle English, ultimately from Old English/West Germanic roots related to 'spear' (Old English 'spere'), where the root referred to a pointed pole or beam.

'spar' appeared in Middle English as forms like 'sparer'/'sparre' meaning a pole or beam used on ships; the verb sense 'to spar' (to strike or box in practice, later to argue) developed later (recorded from the 18th century) from use of the noun and related dialectal senses.

Initially it meant 'a pole or beam' (a physical spar), but over time it developed verbal senses including 'to strike or push with spars', then the figurative sense 'to practice fight' and 'to exchange verbal blows' that are common today.