Flavour, smack. The salt of youth is that vigour and strong passion which then predominates. Shakespeare uses the term on several occasions for strong amorous passion. Thus Iago refers to it as “hot as monkeys, salt as wolves in pride” (Othello, iii. 3). The Duke calls Angelo's base passion his “salt imagination,” because he supposed his victim to be Isabella, and not his betrothed wife whom the Duke forced him to marry. (Measure for Measure, v. 1.)
“Though we are justices, and doctors, and churchmen, Master Page, we have some salt of our youth in us.” —Merry Wives of Windsor, ii. 3.
Spilling salt was held to be an unlucky omen by the Romans, and the superstition has descended to ourselves. In Leonardo da Vinci's famous picture of the Lord's Supper, Judas Iscariot is known by the salt-cellar knocked over accidentally by his arm. Salt was used in sacrifice by the Jews, as well as by the Greeks and Romans; and it is still used in baptism by the Roman Catholic clergy. It was an emblem of purity and the sanctifying influence of a holy life on others. Hence our Lord tells His disciples they are “the salt of the earth.” Spilling the salt after it was placed on the head of the victim was a bad omen, hence the superstition.
A covenant of salt (Numbers xviii. 19). A covenant which could not be broken. As salt was a symbol of incorruption, it, of course, symbolised perpetuity.
“The Lord God of Israel gave the kingdom ... to David ... by a covenant of salt.” - 2 Chronicles xiii. 5.
Cum grano salis. With great limitation; with its grain of salt, or truth. As salt is sparingly used in condiments, so is truth in the remark just made.
He won't earn salt for his porridge. He will never earn a penny.
Not worth one's salt. Not worth the expense of the food he eats. To eat a man's salt. To partake of his hospitality. Among the Arabs to eat a man's salt was a sacred bond between the host and guest. No one who has eaten of another's salt should speak ill of him or do him an ill turn.
“One does not eat a man's salt ... at these dinners. There is nothing sacred in ... London hospitality.” —Thackeray.
To sit above the salt - in a place of distinction. Formerly the family saler (salt cellar) was of massive silver, and placed in the middle of the table. Persons of distinction sat above the “saler” —i.e. between it and the head of the table; dependents and inferior guests sat below.
“We took him up above the salt and made much of him.” —Kingsley: Westward Ho / chap. xv.
True to his salt. Faithful to his employers. Here salt means salary or interests. (See above, To eat a man's salt.
“M. Waddington owes his fortune and his consideration to his father's adopted country [France], and he is true to his salt.” —Newspaper paragraph, March 6, 1893.
A sailor, especially an old sailor; e.g. an old salt.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894