1. slang Behaving in an erratic or unpleasant manner. Oh man, he's really illin'—I have no idea why he just stormed out of here.
2. slang Unhappy. A: "Why's she illin' today?" B: "Well, it looks like she got an F on her psych paper. I bet that has something to do with it."
3. slang Sick or unwell. I was illin' last week, that's why I was absent.
See also: ill