The adjective piping hot is used to refer to very hot food or liquid, usually when served.

It referred originally to the hissing of viands in the frying pan, the verb pipe meaning, in this case, to make a whistling sound.

This adjective is first attested in The Miller’s Tale, by the English poet Geoffrey Chaucer (circa 1342-1400):

(interlinear translation)
He sente hire pyment, meeth, and spiced ale,
    He sent her sweetened wine, mead, and spiced ale,
And wafres, pipyng hoot out of the gleede;
    And wafers, piping hot out of the fire.

In A true & exact history of the island of Barbados, published in 1657, Richard Ligon (circa 1585-1662), British business agent and natural science writer, used piping hot figuratively:

We had climed with infinite difficulty; and indeed so painfull and violent was our motion: (our leggs finding the motion of elevation, much more violent then of distention,) as we were almost scalded within, and the torrid heat of the Sun, being then our Zenith, did so scald us without, as we were in fitter condition to be fricased for the Padres dinner, then to eat any dinner our selves. Being painfully and pipeing hot, arriv’d at this exalted mansion.

By analogy with food that has just been cooked, piping hot is also used to mean just come out, quite new, ‘fresh’. For example, in An almond for a parrat; or, Cutbert Curry-knaues almes (1590), the English pamphleteer, poet and playwright Thomas Nashe (1567-circa 1601) wrote of a “president of funeralls piping hot from the primitiue church”. In the same pamphlet, he wrote:

Looke, looke good people where that vile whooremaster Iohn a Borhead comes in piping hot from Clayphams wife.

The adjective piping is used in the same sense as piping hot. In Dartmoor, and Other Poems (1823), by the English author Joseph Cottle (1770-1853), the poem titled Mr. Body’s Remonstrance With His Dissolute Master, Mr. Mind contains the following:

And now (with grief I tell) comes piping toddy,
Or punch, to torture me afresh, poor body!