is Ros-marinus (seadew), and is said to be “useful in love-making.” The reason is this: Both Venus, the love-goddess, and Rosemary or sea-dew, were offspring of the sea; and as Love is Beauty's son, Rosemary is his nearest relative.
The sea his mother Venus came on; And hence some reverend men approve Of rosemary in making love.
Butler: Hudibras, pt. ii. c. 1.
Rosemary, an emblem of remembrance. Thus Ophelia says, “There's rosemary, that's for remembrance.” According to ancient tradition, this herb strengthens the memory. As Hungary water, it was once very extensively taken to quiet the nerves. It was much used in weddings, and to wear rosemary in ancient times was as significant of a wedding as to wear a white favour. When the Nurse in Romeo and Juliet asks, “Doth not rosemary and Romeo begin both with a [i.e. one] letter?” she refers to these emblematical characteristics of the herb. In the language of flowers it means “Fidelity in love.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894