mul·lein (m?l??n)n. Any of various Eurasian plants of the genus Verbascum of the figwort family, especially V. thapsus, naturalized in North America, having a tall spike of yellow flowers and leaves covered with dense woolly down.[Middle English moleine, from Anglo-Norman, probably from mol, soft, from Latin mollis; see moil.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.mullein (?m?l?n) or mullenn (Plants) any of various European herbaceous plants of the scrophulariaceous genus Verbascum, such as V. thapsus (common mullein or Aaron’s rod), typically having tall spikes of yellow flowers and broad hairy leaves. See also Aaron’s rod[C15: from Old French moleine, probably from Old French mol soft, from Latin mollis]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014mul?lein or mul?len (?m?l ?n) n. any of various plants belonging to the genus Verbascum, of the figwort family, native to the Old World. [1325?75; Middle English moleine