The origin of Pease Porridge, hot and cold, as well as rather old

Everytime I hear “Pease Porridge Hot” I think, “What a terrible advertising jingle.” But then I note that I become desirous for pease porridge of any sort, and I further marvel at the longevity of the ad jingle; as well as the porridge itself.

Pease porridge hot, pease porridge cold,
Pease porridge in the pot, nine days old;
Some like it hot, some like it cold,
Some like it in the pot, nine days old.

I also note that its popularity is possibly spurred by the fact that it long ago fell into the public domain. Now anyone can sing about pease porridge. Or make it in any combination of consistency or temperature. This takes the chill off even the coldest of pease porridge.

[Pease Porridge on Wikipedia, though that page is not as authoritative as this one.]