The word is sterns
Six letter words using the letters 'nuspiegn' are ensign, supine, and genius.
LEGACY
NETHER
iambus
onward
tidies
Versus.
fervor
There are no English words that meet the criteria, even when you repeat the letters (which you need to do to make five letters spell a six-letter word).
alpaca
Kennel
"AGENDA"