howdy vs how-do-you-do

howdy and how-do-you-do both are nouns.

Word	Noun	Adjective	Verb	Adverb
howdy	Yes	No	No	No
how-do-you-do	Yes	No	No	No

As nouns, howdy and how-do-you-do are synonyms defined as:

Other synonyms of howdy include hello, hi, hullo.

howdy (noun)	how-do-you-do (noun)
an expression of greeting	an expression of greeting
	an awkward situation