'apposability' originates from Latin via Old French and Middle English, specifically ultimately from the Latin verb 'appōnere' (ad- 'to' + pōnere 'to place').

'appōnere' changed into Old French forms such as 'apposer' and Medieval Latin/French nouns like 'appositio', then into Middle English 'apposition'/'appose', and eventually into modern English formations such as 'apposable' and the derived noun 'apposability'.

Initially it meant 'to place to' or 'to put near'; over time the sense broadened to refer to the act or property of placing things side by side, and now specifically to the ability or suitability of being placed in apposition.