Green’s Dictionary of Slang

shocked, surprised and thus rendered motionless.

1592		Shakespeare Henry VI pt I III:2: Now where's the Bastard’s braves, and Charles his gleeks? What, all a-mort?
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: To be taken all-a mort; to be confounded, surprised, or motionless through fear.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].
1859		Hotten Dict. of Modern Sl. etc.
1864		Stamford Mercury 30 Dec. 2/3: ‘You took me all A-mort,’ or confouned me.