From Longman Dictionary of Contemporary Englishfast friendsfast friendsliteraryFRIENDLY two people who are very friendly for a long time → fast
Examples from the Corpus
fast friends• Ishmael and Queequeg become fast friends.• The two of them immediately became fast friends.• Within a few years they had become fast friends.• Other volunteers want to become fast friends and cultural advisers.• The two become fast friends and Herbert gently and tactfully instructs Pip in social behaviour appropriate for a gentleman.• They never, however, became hard and fast friends and there was an unspoken rivalry between them. 