The phrase "off one's nut" is used to describe a state of being insane or crazy. It is a colloquialism and may not be suitable for formal settings. However, if the situation demands, you may use some synonyms that can convey the same meaning without being too vulgar. Some of the alternative expressions you could use are "off one's rocker," "out of one's mind," "lost it," "cracked up," "flipped out," "gone bonkers," or "mad as a hatter." Each of these phrases effectively captures the essence of irrationality or madness in different ways, making them appropriate substitutes for "off one's nut".
