'reciprocalness' is a modern English formation created by adding the suffix '-ness' to the adjective 'reciprocal'. 'Reciprocal' ultimately derives from Latin 'reciprocus' (see below).

'reciprocal' entered English via Middle French 'réciproque' and Late Latin 'reciprocus' (from Latin roots 're-' + 'pro-'). The modern English noun 'reciprocalness' is formed later by combining 'reciprocal' + '-ness'.

Originally from Latin the root conveyed the idea of 'returning the same way' or 'alternating'; over time the sense shifted to 'mutual' or 'done in return', and English developed nouns (e.g., 'reciprocity', 'reciprocalness') expressing that quality.