Spoon head was a racial slur used to describe Cardassians. It was a reference to the spoon-like oval crater on the top of Cardassian foreheads and dated back to the Cardassian Occupation of Bajor. Overall, the term was mainly used by Bajorans and Federation veterans of the Federation-Cardassian War. (DS9: "Things Past", "Empok Nor", "Wrongs Darker Than Death or Night")
The term "spoonheads" was used by the DS9 writing staff for years prior to its canon introduction in "Things Past". The reference actually went back to an illustration of a wide-shouldered woman with a spoon-like shape in the middle of her forehead, seen by Michael Westmore in a Ventura Boulevard store and later used as the inspiration for the Cardassian makeup design. Ronald D. Moore put the term into the script for "Things Past". He believed not only did it make sense for the Bajorans during the Occupation to call the Cardassians derogatory names but also that the term "spoonheads" was "a nice in-joke, too." Moore personally enjoyed introducing the show's audience to it. (Star Trek: Deep Space Nine Companion, p. 398)