If the sheets are white, pour hydrogen peroxide directly on the stain.  If you can, fill a bowl with the peroxide and soak the stained area.  Let it sit for 30 minutes or so, but don't let it dry.  Use a small scrub brush, a rough washcloth, or even your fingertips on the stain every 10-15 minutes.  Rinse with cold water and repeat as necessary.  Launder in cold water before use (obviously).  You may have to repeat the process a few times.  It's best to keep the stain wet.  Don't put it in a hot dryer until the stain is gone.  If you need to dry it before the stain is gone, hang it up, lay it out on a table, or use the air fluff setting on your dryer.  

In 1926, a "roundheel" was a boxer that was easily defeated because his heels were so round that he could be easily pushed onto his back by his opponent.  Soon it referred to prostitutes because it was easy to get them on their backs.  It is also used to describe a very gullible person, a pushover.