A colleague was asking yesterday. A bit of googling suggests it's an older name for a 9 x 13 baking pan. But it doesn't show up in dictionaries, and we wondered if it was a regional term.
I had to laugh once at a guy I was dating. He kept talking about a "meat loaf pan". He was 6'5"
and was the SMALL one in his family with five brothers & sisters. Turned out he was calling a 13X9 pan a meat loaf pan. Guess if you have that many big hungry kids it might be!