A "pity yes" is the expression of half-hearted consent a woman gives to a man's request for a date. It's a "yes" to a request for a date, but not one with enthusiasm. A man can evoke a pity yes with the look on his face, or by asking a woman out in a novel way.
Ack! You can't ask someone out over IM! You always have to ask out someone at least via phone. You're much more likely to get a pity yes. Then, if you make a good impression, you turn that into a real yes!
If you're nervous, ask over the phone. You can call for the explicit purpose of asking out. In person, it's hard to find time to mention it. It's awkward to manufacture an encounter just to do it. And it has to be at the end; you can't get shot down and continue that conversation.