Iron doublet, a parson.
Itchland, Ireland.
Jackrum, a licence.
Jack Adams, a fool.
Jack-a-dandy, a little insignificant fellow.
Jack-in-a-box, a sharper or cheat.
Jack-at-a-pinch, a poor hackney parson.
Jacobites, sham or collar shirts.
Jack, a seal.
Jet, a lawyer