Q From Steve Ellis: When I looked up the phrase piping hot in a dictionary, it gave the definition ‘very hot’. When I looked up piping, I didn’t find any such sense. Can you elaborate?
A The sense of piping that’s relevant here is the one for making a musical sound, as by playing the pipes. The idea is that a dish that’s piping hot is one so hot it makes a sizzling or hissing noise, perhaps not closely similar to the sound of the pipes, but at least audible. It’s first recorded near the end of the fourteenth century, in Chaucer’s Canterbury Tales. In the Miller’s Tale it says (in modernised spelling): “Wafers piping hot out of the gleed”, where a wafer is a kind of thin cake, baked between wafer-irons, and gleed is the hot coals of a fire.