A.T. Robertson Commentary Matthew 5:13

"Ye are the salt of the earth: but if the salt have lost its savor, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out and trodden under foot of men." — Matthew 5:13 (ASV)

Lost its savour (μωρανθη). The verb is from μωρος (dull, sluggish, stupid, foolish) and means to play the fool, to become foolish, of salt become tasteless, insipid . It is common in Syria and Palestine to see salt scattered in piles on the ground because it has lost its flavour, "hae tint its tang" (Braid Scots), the most worthless thing imaginable. Jesus may have used here a current proverb.