This expression was used in the 1972 Carly Simon song, "You're So Vain." To understand the meaning, it is helpful to read the full lyrics of the song. Prior to the refrain, the lyric "And all the girls dreamed that they'd be your partner
They'd be your partner, and..."
What was it that Simon was dreaming about? The prospect of a
relationship with subject of the verse. The lyrics were written about someone after the end of a
relationship, several years afterward, in fact. Simon laments that a
relationship was, is, and will continue to be made impossible by the deep character flaws of the subject, most prominently, their vanity.
Understanding this, 'clouds in my coffee' refers to a metaphorical distortion or
illusion. Coffee is black, clouds are white and have patterns. When one sees such things reflected in the surface of a liquid, they are readily identifiable as illusion. In matters of love, illusions are more difficult to discern.
Simon expresses her lament that her belief in a future with this shallow individual was illusory and that it was an illusion shared over the years by several other women, followed by the statement that the subject WOULD think the song was about them, a statement made ironic by the fact the song really is about them as, for once, they would be arriving at a correct conclusion even if for the wrong reasons.
As a simile: "I thought I saw an oasis, but it was a mirage, like clouds in coffee."
As a
metaphor: "My dream was to be a
movie star, but after many years of failure came to
realize that I was seeing clouds in my coffee."
