classic waka stream

If ever my heart
should turn away from you, then
by the god enshrined
at the mirror shrine of Matsura
let me swear my oath to you

Meaning
Should my heart ever turn away from you, then let me swear it by the god of the mirror shrine at Matsura.
Commentary
Part One: Tamakazura

 Even after the Deputy Governor died at his post the party could not return to the capital, and the girl grew to about twenty and became the talk of the district. A local strongman with a large clan in Higo, the Audit Officer, pressed his suit hard, declaring that he would cherish her whatever her defects; and when the women, thoroughly alarmed, replied that she meant to become a nun, he grew only the more anxious and forced his way across into Hizen. Of the nurse's three sons two went over to him, and only the eldest, the Vice Governor of Bungo, held to the late Deputy's dying charge and argued for taking her up to the capital. The Audit Officer came himself, played the man of the capital, talked on and on, and then, wanting a poem as he took his leave, turned it over in his mind for some while and produced this.

 "The god of the mirror shrine at Matsura" is the deity of the Kagami Shrine in Hizen, invoked here as witness to an oath. "Let me swear by" means to call the god's name in pledge. But having said "should my heart turn away," he never says what would follow, and the poem breaks off with the join badly made. Its very clumsiness renders the figure of a provincial soldier trying to pass for a courtier. That he laughs and praises his own work, while beside him the women are frightened out of their wits, is at once the comedy of the scene and its menace.
Author
Murasaki Shikibu
Source
Genji Monogatari
Other