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1570s, from Irish seamrog, diminutive of seamar "clover." Cf. Gaelic seamrag "trefoil."
any of several similar-appearing trifoliate plants-i.e., plants each of whose leaves is divided into three leaflets. Plants called shamrock include the wood sorrel (Oxalis acetosella) of the family Oxalidaceae, or any of various plants of the pea family (Fabaceae), including white clover (Trifolium repens), suckling clover (T. dubium), and black medic (Medicago lupulina). According to Irish legend, St. Patrick, patron saint of Ireland, first chose the shamrock as a symbol of the Trinity of the Christian church because of its three leaflets bound by a common stalk. Wood sorrel is shipped from Ireland to other countries in great quantity for St. Patrick's Day