From Wiktionary, the free dictionary
From Old Swedish ill- (“evil”). Attested since end of the 13th century. Related to illa.
ill-
- evil
- ill- + vilja (“will”) → illvilja (“ill-will”)
- Generic amplifier
- ill- + grön (“green”) → illgrön (“very green”)
- ill- + tjut (“shriek; howl”) → illtjut (“loud shriek”)