knock someone for a loop

Example: “The news of the merger knocked us all for a loop.”

Origin: Dates to the 1920s and most plausibly grew out of boxing, where a fighter hit hard enough to be knocked backward, sometimes even rolling over in the process, was described using exactly this image. Exactly what "loop" itself alludes to is genuinely unsettled: candidates include an aircraft performing a full aerial loop, a person knocked head over heels in a literal circular tumble, a roped calf brought down by a lariat looped around its leg, and even an early roller coaster's looping track, a theory the Oxford English Dictionary itself considers plausible. Whatever the precise inspiration, all versions share the same comic-strip-style image of someone sent spinning by an unexpected, forceful impact — literal in boxing, figurative everywhere else.