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dandelion (n.)

well-known plant of the daisy family found in Europe, Asia, and North America, with a tapering, milky root, producing one large, yellow flower, late 14c., a contraction of dent-de-lioun, from Old French dent de lion, literally "lion's tooth" (from its toothed leaves), a translation of Medieval Latin dens leonis. From Latin dens (genitive dentis) "tooth," from PIE root *dent- "tooth" + leonis, genitive of leo "lion" (see lion). 

Other folk names, like tell-time refer to the custom of telling the time by blowing the white seed (the number of puffs required to blow them all off supposedly being the number of the hour), or to the plant's regular opening and closing with daylight. Other names refer to its diuretic qualities (Middle English piss-a-bed, French pissenlit).