To share, as a guest, whatever happens to be served to eat

Gli invitati che arrivano all'ultimo momento a un buffet, non possono scegliere, ma devono accontentarsi di quel che passa il convento

Late comers to a dinner party cannot choose, but they have to take potluck

Visto che sei qui, se vuoi fermarti a cena sei più che benvenuto, ma dovrai accontentarti di quello che passa il convento

While you are here, you are more than welcome to join us for dinner, but you'll have to take potluck

The early meaning of this term goes back to Medieval times, when a lot of poor families had to stretch their food in order to survive. The housewife would keep an iron pot on an open fire, throw into it any leftovers she had, keeping the pot simmering over a period of a few days and then serve it as a meal. The word luck comes into the picture because someone invited to share the dinner, had to take his chances, not knowing which kind of food to expect.