WordNet sense 1 (somewhat ill or prone to illness):

ailing, indisposed, peaked, seedy, sickly, unwell, peaked, under the weather

WordNet sense 2 (affected by an impairment of normal physical or mental function):

ill, sick

WordNet sense 3 ((`ill' is often used as a combining form) in a poor or improper or unsatisfactory manner; not well):

badly