the word 'unflappable' is a mid-20th century coinage, primarily in british english. it is built upon the informal term 'flap,' which since the early 1900s meant a state of agitation or panic, itself likely onomatopoeic, imitating the sound of something flapping loosely and uncontrollably. the suffix '-able' is added to mean "capable of being," and the prefix 'un-' negates it, creating the sense of "not capable of being agitated." the literal morphemes thus combine to paint a vivid picture of someone who remains calm and cannot be made to "flap" like a loose sail in the wind, even under pressure.

Structure: un (not) + flap (agitation) + able (capable of) un: a prefix of germanic origin (old english) meaning "not," used to give a negative force. flap: a root of imitative origin (middle english), likely from the sound of something loose beating about, conveying a sense of agitation or panic. able: a suffix of latin origin (via old french) meaning "capable of, fit for, tending to."