A metaphor for seman stains or stains from any other secretions produced during sexual activity that can be found a variety of clothing. Usually the wearer is unaware of the presence of these stains.
'Damn it! I just realised I'm wearing an inky shirt'
'I droppped my pen and got ink on my tshirt!', 'Yeah I left your sister with an inky shirt last night Dave,
it's a pain isn't it?'