Not worth a rush. Worthless. The allusion is to the practice of strewing floors with rushes before carpets were invented. Distinguished guests had clean fresh rushes, but those of inferior grade had either the rushes which had been already used by their superiors, or none at all. The more modern expression is “Not worth a straw.”
“Strangers have green rushes, when daily guests are not worth a rush.” —Lilly: Sappho and Phaon.
Friar Rush. Will-o'-the-Wisp; a strolling demon, who once on a time got admittance into a monastery as a scullion, and played the monks divers pranks. (See Friar's Lanthorn.)