It's a typical expression used in Salento (Puglia). It stands for "your dead parents" and it can be used in different situations and in many ways.
-"Li morti toi, muoviti"
-"Dammi un attimo, sto arrivando"
-"Li morti toi, hurry up"
-"Give me one minute, I'm coming"
-"Li morti loro, abbiamo perso la partita"
-"Lo so, hanno barato sicuramente"
-"Li morti loto, we lost the game"
-"I know, the cheated for sure"