fern seedn. The dustlike spores of ferns, formerly believed to be seeds and once thought to have the power of making their possessor invisible.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.fern seed n the minute particles by which ferns reproduce themselves, formerly thought to be invisible. Possession of them was thought to make a person invisible Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014fern? seed` n. the spores of ferns, formerly supposed to have the power to make persons invisible. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.