1580s, from stain (n.) + -less. Related: Stainlessly. Stainless steel is from 1917, a chromium-steel alloy (usually 14% chromium) used for cutlery, etc., so called because it is highly resistant to rust or tarnish.
word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.
<a href="https://www.etymonline.com/word/stainless">Etymology of stainless by etymonline</a>
Harper, D. (n.d.). Etymology of stainless. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/stainless