What does lose (one's) marbles expression mean? Definition of lose (one's) marbles in the Idioms Dictionary. Lose one’s marbles is an idiom that dates to the mid-1800s. That's worth investigation at least, so let's have a go. I don't think he's got his marbles . Definition of lose marbles in the Idioms Dictionary. To be or become mentally deficient, incompetent, or deranged; to become of unsound mind. 1960 , P[elham] G[renville] Wodehouse , chapter XXI, in Jeeves in the Offing , London: Herbert Jenkins , OCLC 1227855 : One's sanity. Lose one's marbles definition is - to become insane. What does lose marbles expression mean? lose marbles phrase. Lose marbles ... lose (one's) marbles. We will examine the definition of the term to lose one’s marbles, where it came … Lose one's marbles definition: If you say that someone has lost their marbles , you mean that their ideas or behaviour... | Meaning, pronunciation, translations and examples How to use lose one's marbles in a sentence. Look it up now! Lose one's marbles definition at Dictionary.com, a free online dictionary with pronunciation, synonyms and translation. Perhaps 'marbles' meant 'mind' or 'wits' before 'lose one's marbles' was coined. Definitions by the largest Idiom Dictionary. lose (one's) marbles phrase. Marbles are, of course, the little glass or metal balls that children use to play the eponymous game. An idiom is a word, group of words or phrase that has a figurative meaning that is not easily deduced from its literal definition. Definitions by the largest Idiom Dictionary.