perluvinir

From perla (“pearl”) +‎ vinir (“friends”), as in, the friendship is as cherished as a pearl.

perluvinir m pl (plural only, genitive plural perluvina)

1. people who are close friends with each other and who get along really well

   Við vorum perluvinir alla barnæskuna. ― We were BFFs during most of our childhood.

2. (by extension) people who get along really well with each other (often describing a temporary situation)

   Þeir voru engir perluvinir. ― They didn't exactly get along.

   Á meðan leiknum stendur erum við perluvinir. ― During the game we get along really well.

This term can not be used in the singular, nor can it be used describe a group knowing someone else ("They are my good friends"), it can only be used to describe a group of people knowing each other ("All of them are good friends").