Green’s Dictionary of Slang

fake jewellery, usu. so well made that it can pass for real; thus do a jar-up v., to sell someone such a piece; jarred up, duped in such a way.

1959		F. Norman in Encounter n.d. in Norman’s London (1969) 66: But there are one or two [words] that I think are worth mentioning: jar up is one of these, because I think it conveys quite a bit of humour. A jar is a piece of imitation jewellery (tomfoolery) that has been so well made that it appears to anyone who isn’t in the know as the real thing; and to do a jar up is to sell some burk a piece. The jar comes when the person who has bought it finds out that it’s a fake.
1979		F. Norman Too Many Crooks Spoil the Caper 165: ‘Ice in the tank?’ ‘Unless I’ve been jarred up.’.