trefoil, n. & adj.
1 any leguminous plant of the genus Trifolium, with leaves of three leaflets and flowers of various colours, esp. clover.
2 any plant with similar leaves.
3 a three-lobed ornamentation, esp. in tracery windows.
4 a thing arranged in or with three lobes.
--adj. of or concerning a three-lobed plant, window tracery, etc.
trefoiled adj. (also in comb.).
ME f. AF trifoil f. L trifolium (as TRI-, folium leaf)
clover, deuce-ace, leash, set of three, shamrock, tercet, ternary, ternion, terzetto, three, threesome, tierce, trey, triad, trialogue, triangle, tricorn, trident, triennium, trihedron, trilogy, trimester, trine, trinity, trinomial, trio, triphthong, triple crown, triple threat, triplet, triplopy, tripod, triptych, trireme, triseme, triskelion, trisul, triumvirate, triunity, trivet, troika
triality, trinity, triunity, three, triad, triplet, trey, trio, ternion, leash, shamrock, tierce, spike-team, trefoil
, triangle, trident, triennium, trigon, trinomial, trionym, triplopia, tripod, trireme, triseme, triskele, triskelion, trisula, third power, cube, cube root, three, triform, trinal, trinomial, tertiary, ternary, triune, triarch, triadie, triple, tri-, tris-, tria juncta in uno.