An onomatopoeic adverb meaning to suddenly or unexpectedly appear or stick out (ぬうっと is a less abrupt variant).
暗闇から猫がにゅっと顔を出した。
A cat suddenly popped its face out of the darkness.
彼はにゅっと現れて、何も言わずに去っていった。
He popped up unexpectedly, said nothing, and left.
木の陰から枝がにゅっと伸びてきた。
A branch thrust out from behind the tree with a sudden movement.
An onomatopoeic adverb describing standing still and expressionless, giving an imposing presence.
門のところに番人がにゅっと立っていた。
A guard stood there motionless and imposing at the gate.
彼女はにゅっと動かず、じっとこちらを見ていた。
She didn't move a muscle and stared at me without expression.
An onomatopoeic adverb meaning to suddenly stand up (from sitting or lying).
授業中、彼はにゅっと立ち上がって窓の方へ行った。
During class, he suddenly stood up and went toward the window.
猫がにゅっと起き上がって伸びをした。
The cat suddenly got up and stretched.