how-do-you-do vs hullo

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

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

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

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

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