Here's why:
* Prefixes are added to the beginning of a word to change its meaning. There's no recognizable prefix attached to "brawny".
* Suffixes are added to the end of a word to change its meaning or grammatical function. While "brawny" ends in "-y", this is considered part of the base word, not a separate suffix. It's more of a characteristic of adjectives in English.
Origin of "brawny"
"Brawny" comes from the Middle English word "brawn," which referred to muscular strength. The "-y" ending in "brawny" simply makes it an adjective meaning "having strong muscles."
So, while "brawny" doesn't have a clear-cut prefix or suffix, it's formed from an older word and uses a common adjective ending.