means The quality of being rigid, inflexible, or difficult to bend or move — whether in a body, a material, or a manner.

from From Old English 'stif,' meaning rigid or unyielding, a word with deep Germanic roots — a cousin of Dutch 'stijf' and German 'steif.' Linguists trace it back further to a Proto-Indo-European root meaning 'to be thick or compressed,' possibly related to Latin 'stipare,' 'to pack tightly' (the same family that gave us 'constipated'). The '-ness' suffix is the plain old English machinery for turning a quality into a noun, so 'stiffness' is simply 'the state of being stif' — a word that has refused to loosen up for over a thousand years.