That is a derived meaning. Originally,
innipúki was a colloquial word coined by children or teenagers and designating someone who preferred staying indoors (engaged in reading or some other solitary activity) to going out to play or socialising with friends. The word is only attested in writing from the 1990s, but it was most certainly in use at least a couple of decades earlier.
About 20 years ago a music festival called Innipúkinn was first held in Reykjavík as a sort of reaction to the so-called
útihátíðir, which are festivals held in various places around the country, most commonly during the first weekend of August, and where the participants are mostly young people who bring their own tents and spend three or four days dancing, drinking and, often, sleeping around. The
Innipúkinn festival is still going strong and was this year headlined by such artists as
GDRN and
Moses Hightower.
As originally used by children the word was mostly derogatory. It still has a somewhat negative connotation, but can also be used neutrally or mildly teasingly to describe a person who is fond of staying at home. Its use by the most recent generations has been influenced by a character called Innipúkinn in a series of books intended to teach young children traffic rules and safety.