[ad. Ger. tarnkappe.]
A magic cap, securing the invisibility of the wearer.
1856 R. A. VAUGHAN Mystics (1860) I. 3 Rings of Gyges, coats of darkness, tarn-caps, and other means of invisibility. 1863 C. M. YONGE Chr. Names II. 312 Siegfried, by means of his tarn cap, invisibly vanquished the Valkyr.
I’ve just sent in an antedate to the OED, from 1825:
from Lays of the Minnesingers Or German Troubadours of the Twelfth and Thirteenth Centuries By Edgar Taylor 1825.
the tarn-cap, or invisible hat
Tarn-cap also appears in Adolf Hitler’s Mein Kampf.
they rapidly pulled the tarn-cap of lies over their ears