Contrary to most of the definitions found here, this is the correct one:
'ILL' : Adjective meaning 'wild' or 'crazy'. Not always derogatory, but used when something may be so outrageous so as to be unruly.
'ILLIN': Adjective or Verb. Lunatic behaviour. See above.
"The guy was drunk - he was Illin'"
"With those green and yellow clothes you look real ill."