noun
Almost always used in the negative phrase まんじりともしない, meaning 'not sleeping a wink' or 'not getting a moment's sleep'. The standalone noun use is rare.
See also: まんじりともしない
昨夜は心配でまんじりともしなかった。
Last night I was so worried I didn't sleep a wink.
adverb taking the 'to' particle
Rare adverbial use describing a fixed, unwavering gaze. Mostly found in literary or older texts.
彼はまんじりと相手の目を見つめた。
He stared fixedly into the other person's eyes.