Jack

Jack. In the British navy the jack is a small union flag, formed by the intersection of St. George’s and St. Andrew’s crosses, usually displayed from a staff erected on the outer end of a ship’s bowsprit. In merchant ships the union is bordered with white or red. Also, a common term for the jack or cross-trees [from which are suspended the sails]. Also, a young male pike, Esox lucius, under a foot in length. Also, a drinking vessel of half pint contents. … Jack, or Jack Tar, a familiar term for a sailor. A foremast man and an able seaman. It was an early term for short coats, jackets, and a sort of coat-of-mail or defensive lorica, or upper garment.

The Sailor’s Word-Book