Well, it is also a diminutive for "weiner" (e.g., weenie roast) and refers to someone who is, shall we say?, less than forceful and/or decisive. ("Why are there so many weenies in the Congress??")

"Weinerhead" was the Christmas-epithet-of-choice for my oldest Great-nephew (who is four years old). "Grandpa Weinerhead" got lots of hugs, I understand.