falla eins og flís við rass

Etymology
Literally ‘to fit as tightly as a chip (metaphorically a thin, close cut) against the ass’. This idiom is relatively recent in the Icelandic language with the oldest example in the  dating from the early 19th century from Guðmundur Jónsson's.

Verb

 * 1)  to fit like a glove, to fit perfectly