Green’s Dictionary of Slang

Josephus rex n.

In phrases

you are Josephus rex [pun on SE abbr. jo + Lat. rex, king]

you are joking.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.