Purchasable comes from purchase + -able; purchase comes from Anglo-French purchaser, Old French porchacier ('to seek to obtain'), from por- ('forth') + chacier ('to chase'). The historical chain includes Old French porchacier, Latin pro, and Latin captiare. The quoted older senses, "to seek to obtain", "forth", and "to chase", supply the semantic anchors for purchasable. The formation is best read as suffixal, compound, and borrowed: the word purchasable carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "able to be bought". English derivatives include purchasability; related English forms include purchase, purchaser, and purchasing. The result is a adj whose etymology preserves both the older form and the later English specialization.