Upright the Nine of Cups is read as satisfaction, comfort and, per its popular name, a wish granted; reversed, as smugness, overindulgence, or a satisfaction that proves shallower than expected once examined.
The wish-card reputation deserves some skepticism as a source claim: it does not appear in Waite's own 1910 text, which gives the more modest concord, contentment, physical wellbeing for the upright card, and it is not part of the Golden Dawn's Book T material either. It is best treated as a later layer of popular cartomantic tradition, genuinely widespread in current practice but no older than the mid twentieth century as far as the printed record shows, and readers who present it as ancient are passing off a relatively recent convention as something ancestral.
Because of its reputation, the card is disproportionately likely to be drawn into questions phrased as wishes or hopes, and experienced readers generally treat this with some caution, since a card's folk reputation can lead a querent to hear more certainty in a reading than the tradition actually supports. The reversed meaning, smugness or a wish that satisfies less than hoped, functions partly as a corrective to the upright card's popularity, a reminder built into the system that comfort achieved is not the same as comfort that was worth having.
Waite's reversed meanings, imperfection, mistakes and defect, are notably harsher and less psychological than the smugness reading now standard.