hessian, n. — Green’s Dictionary of Slang

hessian n.

[SE Hessian, a Hessian mercenary employed by the British during the War of Independence (1775–83); thus a general term of derision. Note also the hessian fly, supposedly imported by the Hessian troops, the larvae of which devastate wheat crops]

(US prison) a Secret Service agent.

1931

Peter Singer ‘The Electric Warden’ Prison Stories Mar. 🌐 ‘The “Hessians” are coming!’ Tully knew what Gangler meant. ‘Hessians’ were Secret Service agents.