The -bilis and -ilis suffixes have given rise to many intriguing English words. Behind impeccable (L impeccabilis) lies the verb peccare (“sin”); behind indefatigable, fatigare (“weary”). The base of inexorable (L inexorabilis) is orare (“beg,” “pray”), so that the adjective means “unable to be prayed away.” The nouns terminus (“end”) and radix, radicis (“root”) underlie the denominatives interminable (L in-termin-abilis) and ineradicable (L in-e-radic-abilis). In fate,fable,fame, and infant, we have met the Latin verb fari, fatus (“speak”); affable (affabilis), now “polite” or “courteous,” originally meant “able to be spoken to” (ad-); ineffable still means “unable to be spoken out” (ex-). An appetite that is insatiable is “unable to get enough” (satis). A dirigible is an airship that can be directed or steered. What would you make of imperceptible and incorrigible?
Don’t confuse incredible and incredulous, which are both derived from credere, creditus (“believe,” “trust”). Whereas incredible (L incredibilis) means “not able to be believed,” incredulous (L incredulus) means “inclined to disbelieve.” If a department store bargain price is incredible, we should probably be incredulous.
Sometimes Latin verb stems could be modified by phonetic changes. Even in classical Latin, “able to move” (movere, motus) was mobilis, E mobile; and “something easily poured” (fundere, fusus) was fut(t)ilis, E futile. By now, you have probably wondered about able itself. It evolved from L habilis (habere), just as ability derived from Latin habilitas. (At this point, would it be futile to offer a little re-hab-il-it-at-ion?)