fern seed

the spores of ferns, formerly supposed to have the power to make persons invisible.

Origin of fern seed

First recorded in 1590–1600

It is never known to fail, is made from a fern-seed that I alone can pluck, and is not for sale at any druggist's!

The Old-Fashioned Fairy Book |Constance Cary Harrison
We steal as in a castle, cock-sure; we have the receipt of fern-seed, we walk invisible.

The Old Inns of Old England, Volume I (of 2) |Charles G. Harper
Nay, I think rather you are more beholden to the night than to fern-seed, for your walking invisible.

How to Know the Ferns |Frances Theodora Parsons
Those who eat of this fern-seed, she remembered as she sank into sleep that night, shall see the fairies!

The Wolves of God |Algernon Blackwood

The first thing to be done was to get rid of the fern-seed which she now regarded as a hateful thing.

The Children's Book of Christmas Stories |Various

fern seed

the minute particles by which ferns reproduce themselves, formerly thought to be invisible. Possession of them was thought to make a person invisible