conceivable adj.

late 13c., conceiven, "take (seed) into the womb, become pregnant," from stem of Old French conceveir (Modern French concevoir), from Latin concipere (past participle conceptus) "to take in and hold; become pregnant" (source also of Spanish concebir, Portuguese concebre, Italian concepere), from con-, here perhaps an intensive prefix (see con-), + combining form of capere "to take" (from PIE root *kap- "to grasp").

Meaning "take into the mind, form a correct notion of" is from mid-14c., that of "form as a general notion in the mind" is from late 14c., figurative senses also found in the Old French and Latin words. Related: Conceived; conceiving.

Nearly all the senses found in Fr. and Eng. were already developed in L., where the primary notion was app. 'to take effectively, take to oneself, take in and hold'. [OED]

common termination and word-forming element of English adjectives (typically based on verbs) and generally adding a notion of "capable of; allowed; worthy of; requiring; 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). It has 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.

1630s, from in- (1) "not, opposite of" + conceivable. Related: Inconceivably; inconceivability. An Old English word for this was unasmeagendlic.