Green’s Dictionary of Slang

to gape, to stare at fixedly.

1738		Swift Polite Conversation 72: Was ever such a Dunce? How well he knows the Town! see how he stares like a stuck Pig!
1795		Sporting Mag. Aug. VI 235/2: March immediately to the marine pavilion of the Prince; there you may gape like a stuck pig with great decency for at least an hour.
c.1822		Earl of Thanet letter n.d. in Maxwell Creevey Papers (1903) II 51: They ask what is to happen. I answer—you will be ruined, and they stare like stuck pigs.
1989		L. Aparvary Legionnaire’s Journey 290: ‘You could at least say ‘boo’ instead of staring at me like a stuck pig!’ .