'Rigid' is Latin for 'stiff' — from 'rigere' (to be stiff). The root of 'rigor' and 'rigor mortis.'
Unable to bend or be forced out of shape; not flexible; strict and unyielding in attitude or approach.
From Latin 'rigidus' (stiff, hard, inflexible, stern, rough), from 'rigēre' (to be stiff, to be numb with cold, to be frozen), from PIE *reig- (to be stiff, to bind). The PIE root *reig- captures the physical experience of stiffening through cold: muscles that lock up, joints that refuse to bend, corpses that stiffen after death. Latin developed the root into 'rigor
'Rigor mortis' — literally 'the stiffness of death' — is the medical term for the stiffening of muscles that begins a few hours after death. The same root gives 'rigorous' (strict, demanding), 'rigor' (severity, exactness), and the less common 'de rigueur' (strictly required by etiquette or fashion, borrowed from French). A 'rigid airship' (like the Hindenburg) had an internal framework, unlike the flexible 'blimp.'