"Rěn bù zhù" literally means "cannot endure/withhold" — it expresses an involuntary, spontaneous reaction that one cannot suppress, such as laughter, tears, sneezing, or a strong emotional impulse. The character 忍 (rěn) means "to endure" or "to restrain," 不 (bù) is the negation particle, and 住 (zhù) here functions as a result complement meaning "to stop" or "to hold back." Together, they form a verb phrase indicating loss of self-control in response to internal feeling or external stimulus.

This phrase is commonly used with verbs ending in -le or -ing (e.g., 笑了, 哭起来, 打喷嚏) and appears after the subject but before the main verb: Subject + 忍不住 + Verb. It carries a neutral-to-slightly informal register and is widely used in speech and writing to convey authenticity and immediacy of emotion.