'apposable' originates from Latin, specifically the word 'appōnere', where the prefix 'ad-' (appearing as 'ap-' before 'p') meant 'to/toward' and the root 'ponere' meant 'to place'.

'apposable' changed from Latin 'appōnere' → Late Latin/Medieval Latin forms (e.g. 'appositus') → Old French 'apposer' (to place to/near) → Middle English 'apposen'/'appose' and eventually formed the modern English adjective 'apposable'.

Initially it meant 'to put or place near/against', and over time this developed into the idea of 'able to be placed side by side' or 'capable of being put in apposition', which is its current sense.