early 14c., "to gather by acquisition, scrape together," especially grains left in the field after harvesting, but the earliest use in English is figurative, from Old French glener "to glean" (14c., Modern French glaner) "to glean," from Late Latin glennare "make a collection," of unknown origin. Perhaps from Gaulish (compare Old Irish do-glinn "he collects, gathers," Celtic glan "clean, pure"). Figurative sense was earlier in English than the literal one of "gather grain left by the reapers" (late 14c.). Related: Gleaned; gleaning.
<a href="https://www.etymonline.com/word/gleaning">Etymology of gleaning by etymonline</a>
Harper, D. (n.d.). Etymology of gleaning. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/gleaning