It can be used as an adjective or a noun. It's main use is to refer to an object that you forgot the name of. Aslo commonly used to refer to a person who is not cool.
" Bro, hand me that ummm.... dooderwanger"
" Your a lil dooderwanger "
This phrase is often used for when you're irritated at someone. However, this phrase could also be used in a positive way depending on the circumstance. This is a famous phrase and has been used by a legend (Gordon Ramsey).
example 1: Stop it amreen you're getting on my 'Yankee Dankee Doodle shit'
example2: What a lovely day, it must be a 'Yankee dankee doodle' day