When a guy has herpes, the clap, chlamydia (or another "burning STD"), or any combination of std's, and sleeps with a girl, the morning after (or shortly after) she will have a burning situation in her nethers. This is known as a "burns donut."
(Next morning after doin tha nasty)
Glen: "Morning babe, want a burns donut?"
Martha: "What's a burns donut?"
Glen: "I have herpes. Burns, don't it?"