Witches (symbolic of women of low status, like gypsies) were "have-nots" in Elizabethan England, the bottom of the Great Chain of Being. Therefore, they were given little food and had to beg often. The sailor's wife is a "have," a woman of slightly higher status who hoards her food.
So, the first witch plans to punish the wife through the husband. Sound familiar? Are these references to Lady Macbeth and Macbeth?
Witches were thought to be able to kill animals (swine), but not humans. As proof of her revenge, she shows the thumb to the other witch. As you know, hand imagery is very important in the play; Macbeth's wife will continually wash the imaginary blood from her hands in Act V.
All in all, the story is a setup for things to come: petty, undeserved revenge is unnatural. And, don't ever hoard chestnuts from a little old lady.
Shakespeare's "Macbeth" opens with the three witches deciding to meet Macbeth on the heath:
In Act I Sc.3 as agreed upon earlier the three witches meet on the heath and await the arrival of Macbeth. While they are waiting for the arrival of Macbeth, they gossip about what each one of them had been doing in the meanwhile. It is then that the first witch says that she saw a sailor's wife eating chestnuts and when she asked for some she refused. So the first witch remarked angrily that the woman's husband is the captain of a ship that has sailed to Aleppo, a city in northern Syria, and that she will use all her witch craft powers and sail quickly to his ship in a sieve! The other witches promise to help her by giving her favorable winds. Once she reaches the ship, she will board it unnoticed by taking the form of a rat. Since no part of the human anatomy corresponds to the tail she can only take the form of a rat - her two arms and legs will be transformed into the four legs of the rat. Once on board his ship, she will cast a wicked spell on the captain. He will become completely dehydrated and will suffer from insomnia. He will slowly waste away and his ship will be tossed about in many storms and although his ship will not be lost in the ocean, he will have a terribly miserable journey: