The viola tricolor. It has a host of fancy names; as, the “Butterfly flower,” “Kiss me quick,” a “Kiss behind the garden gate,” “Love in idleness,” “Pansy,” “Three faces under one hood,” the “Variegated violet,” “Herba Trinitatis.” The quotation annexed will explain the popular tradition of the flower:
Yet marked I where the bolt of Cupid fell: It fell upon a little western flower, Before milk-white, now purple with loves wound, And maidens call it love-in-idleness ... The juice of it on sleeping eyelids laid Will make a man or woman madly doat Upon the next live creature that it sees.
Shakespeare: Midsummer Night's Dream, ii. 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894