köken: Formed, with the suffix -ible, from Latin ostēnsus, past participle of ostendō (“to show”), itself from obs- (“in front of”) (akin to Latin ob- (“in the way”), Ancient Greek ἐπι- (epi-, “on, at, besides, after”), Old English eofot (“crime”)) + tendō (“to stretch”); or borrowed from Medieval Latin ostēnsibilis.
köken: Inherited from Old Spanish falso, from Latin falsus, with preservation of /alC/ and initial /f/, which Coromines & Pascual suppose is the result of learned pronunciation habits.