"Cecil Jacobs is a big wet he-en." (p. 263). What is meant by "he-en"?
This phrase can be found in Chapter 28. It is spoken by Scout. She and Jem are on their way home from the Halloween pageant and are very nervous because of sounds that they think they hear in the dark. They believe that the sounds are being made by Cecil Jacobs. Cecil scared them once, earlier in the evening, and they think that he is trying to do it again.
As I understand it, "he-en" simply means "hen." The word is sort of drawn out to make it sound more like Scout is making fun of him. It makes sense to me because I can see where calling someone a wet hen would be annoying (we're talking about little kids here...). If you've ever seen a chicken get wet, you'll know that they look pretty ridiculous and they are very unhappy. Presumably everyone in Maycomb would have seen this happen and so this woudl be a much more effective insult than it would be in our time and plae.
I assume in reading this you thought "he-en" was a word in which a letter was left out of the middle or something. If you had heard this part read aloud, it would have been a little easier to understand. Scout is simply calling Cecil Jacobs a "big wet hen." She's being an obnoxious little kid. She thinks Cecil is sneaking up behind them in the darkness and calls this to show him that they know he's there - but because she yells it, she drags out the he-en - not just for emphasis, but also, likely, because of her cute little Alabama accent. I actually imagine it sounded something like "HEE-in."