The word "thin" has numerous opposites: wide, thick, broad, fat, stout, hefty, and substantial; and the verbs thicken, widen, broaden, repopulate, replenish, compress, or concentrate.
Conspicuously absent are antonyms that begin with the letter N. The closest might be natural for unthinned growth. Or the trivial "not thin."
It is possible that this is asking for the synonym, not the antonym.
(A close synonym is narrow.)
a. & n. from Start, v.
Narrow is the opposite of wide. It begins with the letter N.
Respectful is an antonym for mean. It begins with the letter R.
its a nonpolar
Its καχυποψία (kachipopsia) (n.)
Need is another word for require and begins with an N.
N- 5 letters:nannynasalnavelnoiseneedynylonnorthniecenastynoosenannyninthniftynoblenovelnaivenakednastynativenethernervenevernichenexusninnyninjanightnotchnudgenurse
Actually, the word would be nerdy.
nugget
Another name for abnormal swelling starting with 'N' is a Nodule.
norm
nestling