Word parts back + side Pronunciation IPA (key): /ˈbækˌsaɪd/ Audio (US)(file) Hyphenation: back‧side Noun Singular backside Plural backsides (countable) Something's backside is the side that is opposite of the front. (countable) Someone's backside is their buttocks or butt. Synonyms: buttocks, butt and ass The child was spanked in the backside for misbehaving. Wikiwand - on Seamless Wikipedia browsing. On steroids.Remove ads