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.
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Definitions of gleaner
someone who picks up grain left in the field by the harvesters;
someone who gathers something in small pieces (e.g. information) slowly and carefully;