Boo

[Gerald Cohen]

   I'm not sure if this was mentioned in the discussion of "boo" (=nothing).
I had often heard this word used in an expression to describe someone
as overly timid: "He wouldn't say boo to a ghost." I suppose the
original idea was that if you say "boo" to a ghost, the ghost would
become frightened and disappear. Then by shortening; "He wouldn't say
boo."
    "Diddly-boo" is probably a blend of "diddly-squat" and "boo"
rather than the starting point of "boo" (=nothing).

---Gerald Cohen