# GNU `sed` 17 bytes

```sh
/\bill[aiouy]/!q1
```
Explained:
```sh
/                    # Start pattern
 \b                  # Match word boundaries
   ill               # Match literal "ill" as start of a word
      [aiouy]        # Match any of [aiouy] directly after
             /       # End pattern
              !      # If no match is found
               q1    # Exit and set status to 1, OK exits are 0 as per POSIX
```