Origin and history of spendable

"to pay out or away, deprive oneself of" (money, wealth), Middle English spenden, from Old English -spendan (in forspendan "use up"), from Medieval Latin spendere, a shortening of Latin expendere "to weigh out money, pay down" (see expend) or possibly of dispendere "to pay out" (see dispense). The word was borrowed generally in Germanic: Old High German spendon, German and Middle Dutch spenden, Old Norse spenna.

The Middle English word is also probably in part from or merged with Old French despendre, from Latin dispendere.

In reference to anything of exchangeable value (labor, thoughts, time, etc.), "consume, use up," attested from c. 1300. The notion of "consume or use wastefully or fruitlessly" is by late 14c. The intransitive sense "exhaust, wear (oneself) out" is from 1590s (see spent).

common termination and word-forming element of English adjectives (typically based on transitive verbs) with the sense "capable; liable; allowed; worthy; requiring; or bound to be ______ed," sometimes "full of, causing," from French -able and directly from Latin -abilis.

It is properly -ble, from Latin -bilis (the vowel being generally from the stem ending of the verb being suffixed), and it represents PIE *-tro-, a suffix used to form nouns of instrument, cognate with the second syllables of English rudder and saddle (n.).

A living element in English, used in new formations from either Latin or native words (readable, bearable) and also with nouns (objectionable, peaceable). Sometimes with an active signification (suitable, capable), sometimes of neutral signification (durable, conformable). By 20c. it had become very elastic in meaning, as in a reliable witness, a playable foul ball, perishable goods. A 17c. writer has cadaverable "mortal."

To take a single example in detail, no-one but a competent philologist can tell whether reasonable comes from the verb or the noun reason, nor whether its original sense was that can be reasoned out, or that can reason, or that can be reasoned with, or that has reason, or that listens to reason, or that is consistent with reason; the ordinary man knows only that it can now mean any of these, & justifiably bases on these & similar facts a generous view of the termination's capabilities; credible meaning for him worthy of credence, why should not reliable & dependable mean worthy of reliance & dependence? [Fowler]

In Latin, -abilis and -ibilis depended on the inflectional vowel of the verb. Hence the variant form -ible in Old French, Spanish, English. In English, -able tends to be used with native (and other non-Latin) words, -ible with words of obvious Latin origin (but there are exceptions). The Latin suffix is not etymologically connected with able, but it long has been popularly associated with it, and this probably has contributed to its vigor as a living suffix.