around the bend

1.  Around a curve or corner on a road or pathway, as in Peter's house is just around the bend. Also see around the corner, def. 1.
2.  Also, round the bend. Crazy, insane, as in Throwing out that perfectly good steak? Have you gone round the bend? [Colloquial; early 1900s]

The adjective has one meaning:

Meaning #1: informal or slang terms for mentally irregular
  Synonyms: balmy, barmy, bats, batty, bonkers, buggy, cracked, crackers, daft, dotty, fruity, haywire, kooky, kookie, loco, loony, loopy, nuts, nutty, round the bend, wacky, whacky