Literal translation
from one olive at the backside, he makes you a liter of oil
Meaning
he's terrified, a real coward
Explanation
He's very scared, a real coward. Someone afraid clenches their buttocks, a common image, but here it's pushed to imagining that a single olive slipped behind would come out pressed into a liter of oil, the clenching is that strong. From this image comes the word fadòli (literally makes oil), which passed into regional French to mean a coward, before its resemblance to fada shifted its meaning toward someone a bit simple.