A person in your life you can't get rid of or deny knowing. Other aspects of your life are fine, but there lies a big old reminder and someone is always pointing it out. Every time you look at him/her, you wish your life was stain-free.
-noun
1. a person, place, or object which has caused annoyance or embarrassment 2. a part of the psychic apparatus that experiences and reacts to the outside world and thus mediates experiences of discomfort, hardship, or embarrassment, most commonly through alcohol
-verb
1. to take advantage
Those guys were stains.
I drank fifteen beers last night; my stain is huge right now.
I stained that girl out last night.