Definition of banana slug
: any of a genus (Ariolimax) of large slugs native to moist forests of western North America that have a long, fusiform body and are sometimes bright yellow in color with brown or black spots … on a pile of redwood stumps by the side of the road, we discovered banana slugs, curled-up balls of bright-yellow ooze that slowly expand to full banana size, seven inches or more, as they glide their slimy way around. — Tamar Lewin, New York Times, 18 Aug. 1996 Banana slugs prefer to graze on wild plants, fallen leaves or, in rare cases, dead animals. — Scott McCredie, Smithsonian, February 1989
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First Known Use of banana slug
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