Very hot, usually referring to food.
In Scotland, ceremonial dishes of food are often brought to the table to the accompaniment of bagpipes, i.e. they are 'piped in'. This could easily be imagined to be the origin of 'piping hot'. It isn't though. Nor does the phrase derive from food being 'piped aboard' ships. The derivation of this little phrase is the sizzling, whistling sound made by steam escaping from very hot food, which is similar to the sound of high-pitched musical pipes.
An early citation is given in Philemon Holland's translation of Pliny's Historie of the world, 1601:
"Beanes... fried all whole as they be, and so cast piping hot into sharp vineger."
Chaucer had also used the phrase, in language less accessible to us, but much earlier. This is from The Miller's Tale, circa 1390:
He sente hir pyment meeth and spiced ale
And wafres pipyng hoot out of the glede.
[He sent her sweetened wine and well-spiced ale
And waffles piping hot out of the fire]