Rigid

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late Middle English: from Latin rigidus, from rigere ‘be stiff’.

wiktionary

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From Middle English rigide, from Latin rigidus(“stiff”), from rigeō(“I am stiff”). Compare rigor. Merged with Middle English rigged, rygged, rugged(“upright like a spine, rigid”, literally “ridged”), from ridge +‎ -ed.

etymonline

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rigid (adj.)

"stiff, unyielding, inflexible, firm, not easily bent," early 15c., from Latin rigidus "hard, stiff, rough, severe," from rigēre "be stiff" (from PIE root *reig- "stretch; be stretched; be stiff"). Related: Rigidly. As a verb, "to make rigid," rigidize is attested by 1944 in U.S. military and commercial use in reference to metals; earlier was rigidify (1842), rigidification.