Chickens, for some reason still known only to chickens, usually stretch out their necks when put on the chopping block, making it all the easier for the butcher to chop their heads off. Probably our expression warning someone not to expose himself to danger or criticism when this can be avoided, which is American slang from the late 19th century, originated from the bloody barnyard image. Lynchings have also been suggested, but men to be lynched rarely stick their necks out for the noose.