Look in the OED & you'll find "brrrr", with a rare 4 Rs: it's this one of the traditional 8 parts of speech
...... Where among mod- ern writers can you nd their superiors in clearness and
... [DOC] [TLE] Guide to Patterns and Usage in English (Second Edition) - Scribd [PAR] It scts out and dcscribu the various parts of speech.