Meaning

noun any of several Old World herbs of the genus Medicago having small flowers and trifoliate compound leaves

noun a plant of the genus Trifolium

noun an architectural ornament in the form of three arcs arranged in a circle

originFrom Middle English trefoil, from Old French trifoil, trefeul, from Latin trifolium, from tri- (“three”) + folium (“leaf”).

Etymology from Wiktionary, CC BY-SA 3.0