fundament

Definition

The rump, the backside. See ass for synonyms.
Etymology: Lawrence Paros. The Erotic Tongue (1984): ' Our Fanny derives from fundament (13 ^th C. for bottom), and she was shortened and punned successfully into funniment, fun, funny, and at last Fanny. But there was nothing funny about her problems in the States. As late as 1929, Fanny found herself on B.F. Keith's list of forbidden expressions that vaudeville performers were not allowed to use on the stage. Instead, they preferred the part that went over the fence last, the caboose, and where the sun never shines. '