'discountable' originates from modern English, built from the verb 'discount' + the adjectival suffix '-able'. 'discount' itself comes from Old French 'descompter' (or Medieval Latin 'discomputare'), where 'dis-' meant 'apart' or 'away' and 'computare' meant 'to calculate'.

'discount' changed from Latin 'discomputare' into Old French 'descompter'/'descounter' and then entered Middle English as 'descounten'/'discounten', eventually yielding the modern English verb 'discount'; the adjective 'discountable' formed later by adding the suffix '-able'.

Initially, the root meant 'to calculate or subtract' (i.e. to reckon a deduction), and over time its sense narrowed toward 'to reduce price' so 'discountable' now means 'able to be reduced in price or eligible for a discount'.