'google' yielded no answers, but...
since it is considered a main dish, (savory bread pudding, or otherwise, an egg casserole), utilizing - day or two, old bread, eggs and the addition of (whatever) meats, cheeses and/or vegetables...
it's logical to assume that it's origins' are rooted in obscure, italian, 'peasant cuisine'.
btw... someone else was stumped for an answer (to your same query), on a site called 'hungrybrowser, back in 2002.