Verb
(UK, colloquial) To cease to behave in a consistent and/or rational manner.
(UK, colloquial) To lose sight of an important objective or principle; to act contrarily to one's own interests through concentrating on relatively unimportant matters.
Used other than figuratively or idiomatically: see lose, plot.
Source: en.wiktionary.orgI'm very close to my family. Not like these big stars - not mentioning any names - who lose the plot and don't know who they are. Jennifer Ellison