Green’s Dictionary of Slang

ill adj.

[fig. uses of SE ill, unwell]
(US black/teen)

1. [1900s; 1980s+] angry, frustrated; of an animal, vicious.

2. [1970s+] aggressive, offensive, bad.

3. [1980s+] (also illy) on the bad = good model, wonderful, first-rate.

4. [1990s+] unattractive.

In exclamations

that’s so ill!

[1980s+] (US teen) an all-purpose denunciation of an object or activity.

SE in slang uses

In compounds

ill fortune (n.) [unfortunate in that it is not a whole shilling]

[late 17C–early 19C] ninepence (a coin).

ill music (n.)

see separate entry.

ill willie (adj.) [SE ill will]

[20C+] (Ulster) uncooperative.