It’s about time. We are now on Instagram!
1550s, "retardation," from Middle French retardance, from retarder (see retard (v.)). In reference to resistance to fire, 1948.
1640s, from retard (v.) + -ant or from Latin retardantem (nominative retardans), present participle of retardare. From 1952 as a noun, "retardant substance."