When a girl has a great body but a gigantic ass. Girls who sit at an office desk all day tend to have all the fat on their body gravitate toward the ass-ular region, hence-office ass.
That girl sitting in cubicle 7 was so fine until she stood up and revealed a case of office ass so severe, it looked like two baby elephants were being born in her jeans.
A piece of ass that you would only consider hitting because it is at work and there isn't much else to compare it to. Outside of work, you probably wouldn't look at twice, unless you've had several beers.
I saw Kristen from accounting out the other night, and she doesn't look that good in a bar full of other hot chicks. I think she's just office ass.