Something or someone that sticks (pierces, or adheres).
[…] and I said to Mr. McCauley, whatever is the matter with your neck, it is all swelled up, and he said he got it in the mill; he said he had an arm load of wool and a sticker stuck him in the neck, here (indicating the right hand side of the neck just below the jaw).
The prisoner fell flat on his face. They dragged him again, this time towards the grove of calafates. They lifted him up there and they threw him in the middle of the bushes. The boy screamed. Thousands of stickers pierced into his flesh.