mugwort (n.)

a popular name for a tiny two-winged fly, applied indiscriminately to many small insects, Old English mygg, mycg "gnat," from Proto-Germanic *mugjon (source also of Swedish mygga, Old Saxon muggia, Middle Dutch mugghe, Dutch mug, Old High German mucka, German Mücke "midge, gnat"). No certain cognates beyond Germanic, unless doubtful Armenian mun "gnat" and Albanian mize "gnat" are counted. Watkins, Klein and others suggest an imitative root used for various humming insects and a relationship to Latin musca "fly" (see mosquito). Meaning "diminutive person" is from 1796.

"a plant," Old English wyrt "root, herb, vegetable, plant, spice," from Proto-Germanic *wurtiz (source also of Old Saxon wurt, Old Norse, Danish urt, Old High German wurz "plant, herb," German Wurz, Gothic waurts, Old Norse rot "root"), from PIE root *wrād- "branch, root." St. John's wort attested from 15c.