Mad as hops
Q From Michael Turniansky: In Patchwork Girl of Oz (L Frank Baum, 1913), I came across the expression mad as hops. Is that the forerunner of hopping mad? And what is so mad about hops?
A There are indeed a number of old expressions using the word hops in the sense of the plant. One was as thick as hops, referring to the dense mats of creeper you can get when hops grow wild and unchecked, and another was as fast as hops, because the plant dies back in winter and then grows very rapidly from its base each spring. These were both known by 1630 at the latest. The phrase hopping mad also dates from the seventeenth century, but it uses a different sense of hop, the one of jumping up and down on one foot, in other words of being so angry that one is literally unable to keep still. It seems that sometime in the nineteenth century in the US somebody punned by combining these phrases to create as mad as hops. The expression is first recorded in Harper’s Magazine in 1884, but is probably older. So there’s nothing mad about the plant at all.