I'll be neck

This means the same as the phrase " I'll be back " or " I'll return ". Neck is being used as a synonym or substitute for Return. If back is being used a as a synonym for return, neck should be considered equivalent because the neck area is on the posterior side of your body. The throat area is in the front and the neck is on the back of the body. Neck, back and your rear end (butt) are all on the posterior side of your body, they are all on the equivalent side of your body. Therefore they are all synonyms for return.