[“Coin operating coffee machine with 4 possible mixtures, each selling for five cents.” Photograph by Wallace Kirkland. February 1947. From the Life Photo Archive. Click for a larger view.]
Why piping hot? “Because of the whistling sound made by very hot liquid or food,” says the Oxford English Dictionary. Its first citation is Chaucer’s “Miller’s Tale” (c. 1390):
He sente hir pyment meeth and spiced aleIn other words:
And wafres pipyng hoot out of the glede.
He sent her honeyed wine, mead, and spiced ale,Also some coffee with cream and sugar, piping hot out of the machine.
And cakes, piping hot out of the fire.
[The OED gives pipinge and pipeinge as v.rr., variant readings, for pipyng. The ersatz Chaucer is mine.]