Cressid, often written Cressida, Cresseid or Criseyde, first appeared in a work by the 12th Century poet Benoît de Sainte-Maure.
A young Trojan woman living during the Siege of Troy, she is in love with Troilus, the youngest son of King Priam. But whilst a hostage in the hands of the Greeks, she is seduced by Diomedes, a Greek warrior.
Cressida is sometimes seen as symbolising inconstancy in love. Whether Waugh wishes us to equate Julia with Cressida is not clear.