"Shepherd's Hook" and, less commonly "Shepherd's Crook" are common names for the long, coiled retaining wire inside the FCG (fire control group) of the *AK-47 family of assault rifles. The wire's purpose is to fasten the front and rear axis pins of the trigger and disconnecter inside the receiver. It is notorious for being difficult to put in and, if improperly installed, can result in major malfunctions when firing.
A Shepherd's Hook may be replaced with an aftermarket retainer plate.
* "The AK-47 family" includes the original Russian AK-47, the AKM: it's more common lightweight variant, the RPK: its heavy-duty machine gun variant, and the more modern 5.56mm AK-74 series of rifles. Also includes the vast majority of non-Russian derivatives.
"Well, boss, I was gonna go cap this mothafucka, right, but the goddamn trigger went all crooked an shit and wouldn't pull. Fuckin towelhead that made this bitch put the Shepherd's Hook in upside down."