This is the 361st most frequent Icelandic word.
illa
Badly.
Here, 'illa' is used as an adverb to describe the manner in which she slept.
Hún svaf illa í nótt.
She slept poorly last night.
In this sentence, 'illa' functions as an adverb describing his state of health or well-being.
Honum líður illa.
He feels unwell.
Here, 'illa' modifies the verb to describe the manner of the dog's behavior, emphasizing a negative attribute.