As she turns down the bed to await her husband, Othello, Desdemona recalls for Emilia how she heard and learned "a song of 'Willow'," from her mother's maid, Barbary:
My mother had a maid call'd Barbary:
She was in love, and he she loved proved mad
And did forsake her: she had a song of "Willow,"
An old thing 'twas, but it express'd her fortune,
And she died singing it. That song tonight
Will not go from my mind; I have much to do,
But to go hang my head all at one side,
And sing it like poor Barbary. (4.3.26-33)
In the same way that "poor Barbary" found in the lyrics of the song a perfect match to her lovelorn spirit, so too does Desdemona. Like Barbary, Desdemona is losing her beloved husband as a result of a kind of 'madness' bred out of an all-consuming jealousy. Presumably Barbary died of a broken heart. Soon after the lyrics leave her lips, Desdemona will die of that cause as well as by the hands of her avenging husband.
Depressed, engulfed in foreboding, Desdemona's lost love is evoked not only by the subject of the song itself, but by the image of the 'weeping willow', a tree thought to symbolize a rain of tears, or a person (likely a maiden) hanging her hair in mourning. Thus, the words of the song speak of a "poor soul", reclining by a "sycamore tree", weeping and moaning, her salt tears falling into "fresh streams".
As she concludes the first verse of the song, Desdemona, unconsciously, adds a line of her own - "Let nobody blame him; his scorn I approve," - singing now of her own feelings, and not those of the sad woman of the song. The embittered words of the second verse - "If I court moe women, you'll couch with moe men." - mirror the destiny of the hapless Desdemona. Inasmuch as the "mad" lover of the song made his unfaithfulness his lady's fault, so too has Othello done likewise with his wife, thus sealing her doom.