the word 'undergird' is a compound formed from two old english elements, 'under' and 'gyrdan' (to gird). the literal sense of 'girding from beneath'—providing foundational support by encircling or binding from below—was originally a nautical term for securing a ship's hull with ropes or chains to prevent it from breaking apart in heavy seas. this concrete, physical meaning evolved into the modern abstract sense of providing fundamental support, strength, or a basis for an idea, system, or structure. the logic of the word perfectly captures the transition from a tangible act of reinforcement to a metaphorical one, where principles or evidence 'undergird' an argument.

Structure: under + gird under (old english; meaning 'beneath, below'), gird (old english; meaning 'to encircle, bind, or secure')