To give one a sneck posset is to slam the door in his face (Cumberland and Westmoreland). The “sneck” or snick is the latch of a door, and to “sneck the door in one's face” is to shut a person out. Mrs. Browning speaks of “nicking” the door.
“The lady closed That door, and nicked the lock.” Aurora Leigh, book vi. line 1,067.
Probably allied to niche, to put the latch into its niche.