from
The Collaborative International Dictionary of English v.0.48
Shamrock \Sham"rock\, n. [L. seamrog, seamar, trefoil, white
clover, white honeysuckle; akin to Gael. seamrag.] (Bot.)
A trifoliate plant used as a national emblem by the Irish.
The legend is that St. Patrick once plucked a leaf of it for
use in illustrating the doctrine of the trinity.
[1913 Webster]
Note: The original plant was probably a kind of wood sorrel
({Oxalis Acetocella}); but now the name is given to the
white clover ({Trifolium repens}), and the black medic
({Medicago lupulina}).
[1913 Webster]