The term SUFFIX FOR 'MOUNTAIN' OR 'MUSKET' can be solved by two different crossword solutions: EER (3 letters) and EERS (4 letters). The suffix 'EER' is derived from the Old French word 'er', which means 'one who does something'. This suffix is commonly used to form nouns that refer to a person who performs a particular action or job. For example, a 'MOUNTAINEER' is someone who climbs mountains, and a 'MUSKETEER' is someone who uses a musket. The suffix 'EERS' is simply the plural form of 'EER'.