old man's pepper

Compound of old +‎ man's +‎ pepper, presumably so-called for its pungent flavor (see quotations below).

old man's pepper (uncountable)

1. Achillea millefolium or common yarrow, a flowering plant native to temperate regions of the Northern Hemisphere.
   • 1852, Anne Pratt, “Common yarrow or milfoil”, in Wild Flowers, Volume 1‎, page 79:

     It’s foliage is slightly pungent, and hence its familiar name of Old Man’s Pepper. The Icelandic appellation of this plant, Vall humall, Field hop, seems to imply that it has been used instead of hops in that island, as it still is in some parts of Sweden.

   • 1857 September, “Rambles around Bristol”, in Bristol Magazine and West of England Monthly Review‎, page 248:

     The leaves are pungent and were formerly mingled with colder herbs as a seasoning; Hence called “Old man’s pepper.”

• “old-man’s-pepper”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.