Green’s Dictionary of Slang

half-cocked adj.

drunk.

[UK] Gent.’s Mag. Dec. 559/2: To express the condition of an Honest Fellow [...] under the Effects of good Fellowship, it is said that he is [...] 45. Half cocked.
Beckett Paradise Lost 19: Half-cocked with swigging ale and beer [F&H].
R.S. Surtees Hunting Field 15: Party merry. Pigskin half-cocked [...] Pigskin three parts drunk; Master half-cocked.
[US]Kalida Venture (OH) 11 Apr. 2/4: Drunk [...] half-cocked.
H. Smart Saddle and Sabre Ch. xvii: ‘Black Bill’ [...] was very often half-cocked when he got up to ride... The man could ride as well half-drunk as sober [F&H].
[US] ‘Word-List From West Brattleboro’ in DN III:vi 453: half-cocked, adj. half drunk.
[US] ‘Sl. Expressions for Drunk’ in New Republic in AS XVI:1 (1941) 9 Mar. 70/1: half-cocked.
[UK]‘William Juniper’ True Drunkard’s Delight.
C. Merrigan ‘Gleaner’s Union’ in ThugLit Sept./Oct. [ebook] [W]e was half-cocked, in no kind of condition to go tracking in the dark.

In phrases

go off half-cocked (v.)

to express unguarded emotion; to lose one’s temper.

[US]Dayton Dly Empire (OH) 27 Dec. 1/4: Sherman had gone off half-cocked and the excitement exploded in good humor.
[US]C.B. Yorke ‘Snowbound’ in Gangster Stories Oct. n.p.: ‘Thanks, Queen, for not letting me go off half-cocked’.
[Aus]C. Bowles G’DAY 38: Doan go off arf-cocked like yer always do.
[Aus]G. Disher Crosskill [ebook] ‘[N]o need to go off half-cocked’.
[UK]C. McPherson Port Authority 5: You can’t ever go off half-cocked, half-medicated into unchartered waters, you’ll be eaten alive.
[Aus]C. Hammer Silver [ebook] He needs to learn his lesson: no more going off half-cocked.
[Aus]C. Hammer Opal Country 212: The last thing he needs is to go off half cocked.