lowly vs petty

lowly is not a noun while petty is a noun.

Word	Noun	Adjective	Verb	Adverb
lowly	No	Yes	No	No
petty	Yes	Yes	No	No

As adjectives, lowly and petty are synonyms defined as:

Other synonyms of lowly include junior-grade, lower-ranking, secondary, subaltern.

lowly (noun)	petty (noun)
	larceny of property having a value less than some amount (the amount varies by locale)

lowly (adjective)	petty (adjective)
low or inferior in station or quality	inferior in rank or status
inferior in rank or status	contemptibly narrow in outlook
of low birth or station (`base' is archaic in this sense)	(informal) small and of little importance
used of unskilled work (especially domestic work)