A quick bath or shower where one washes one's self except for the hair. It is usually very quick, up to about 5 minutes.
Also, not necessarily done before or after sex. A primary reason for a spit bath might be because your hair isn't dirty yet, so there's no point in washing it. May also be used to simply freshen up.
After a long day at work, Susan wanted a spit bath before heading out with the girls.