Definition from Wiktionary, the free dictionary
From Middle English malliable, borrowed from Late Latin malleābilis, derived from malleāre (“to hammer”), from malleus (“hammer”), from Proto-Indo-European *mal-ni- (“crushing”), an extended variant of Proto-Indo-European *melH₂- (“crush, grind”).
- Able to be hammered into thin sheets; capable of being extended or shaped by beating with a hammer, or by the pressure of rollers.
- (metaphorical) Flexible, liable to change.
- My opinion on the subject is malleable.
- (cryptography, of an algorithm) in which an adversary can alter a ciphertext such that it decrypts to a related plaintext
Related terms 
Coordinate terms 
able to be hammered into thin sheets
liable to change