When you jilt someone, you throw them over or reject them, especially romantically. It would be cruel to jilt your boyfriend on the day before the prom.
If a bride jilts her groom, she leaves him waiting for her in his tuxedo, and if a boy jilts his date, he never shows up at the planned time. In either case, it's an unkind and thoughtless thing to do to someone. In the 1660s, to jilt meant "to deceive, cheat, or trick," and it was also a noun meaning "woman who gives hope and then dashes it," from the Middle English root gille, "lass or wench."