The term 'guy, informally' can be expressed in two different ways: 'FELLA' (5 letters) and 'FELLER' (6 letters). Both terms are used to refer to a man in an informal way. FELLA is derived from the Old English word 'feolaga' which means companion or associate. FELLER is derived from the Middle English word 'felawe' which also means companion or associate. Both terms are used to refer to a man in a casual or friendly manner.