The word "忍俊不禁" is formed from four characters: "忍" meaning to endure or restrain, "俊" which in this context historically contributes to the sense of smiling or laughing, "不" meaning not, and "禁" meaning to prohibit or hold back. The phrase originates from classical Chinese where "忍俊" together denotes smiling or laughing, and when combined with "不禁" (cannot help), it literally expresses the idea of being unable to restrain laughter, thus forming an idiom meaning cannot help but laugh.