the word untenable is a direct borrowing from french 'intenable', which itself is built from latin components. the core is the latin verb 'tenēre', meaning "to hold, keep, possess," a verb with a robust history traceable to the proto-indo-european root ten- meaning "to stretch." the prefix 'un-' (from old english) was added to the french base to create a negative sense. the literal, physical meaning of "not able to be held or occupied" (as in a military position or a building) evolved logically into the modern abstract sense of an argument or position that cannot be intellectually maintained or defended against criticism. thus, the journey from the physical act of holding to the metaphorical act of defending an idea is complete.

Structure: un (not) + ten (hold) + able (capable of) un- (old english, a prefix meaning "not"); ten- (latin, from 'tenēre', meaning "to hold"); -able (latin/french, an adjective-forming suffix meaning "capable of, fit for").