how-do-you-do vs hi

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

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

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

Other synonyms of how-do-you-do include hello, howdy, hullo.

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