Yes, the ordinal number seventh (7th) can be an adverb. It can also be an adjective (e.g. seventh child) or a noun (as for the fraction one seventh).
*The other adverb form "seventhly" is very seldom seen.
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According to SOWPODS (the combination of Scrabble dictionaries
used around the world) there are 1 words with the pattern
---EN-HL-. That is, nine letter words with 4th letter E and 5th
letter N and 7th letter H and 8th letter L. In alphabetical order,
they are:
seventhly
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According to SOWPODS (the combination of Scrabble dictionaries
used around the world) there are 7 words with the pattern
---E-TH--. That is, nine letter words with 4th letter E and 6th
letter T and 7th letter H. In alphabetical order, they are:
nepenthes
seventhly
spreathed
spreathes
spreethed
spreethes
superthin
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According to SOWPODS (the combination of Scrabble dictionaries
used around the world) there are 7 words with the pattern
-----THL-. That is, nine letter words with 6th letter T and 7th
letter H and 8th letter L. In alphabetical order, they are:
bimonthly
pentathla
seventhly
twelfthly
uncouthly
unearthly
youngthly
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According to SOWPODS (the combination of Scrabble dictionaries
used around the world) there are 15 words with the pattern
S---N---Y. That is, nine letter words with 1st letter S and 5th
letter N and 9th letter Y. In alphabetical order, they are: