不由 is an adverb used to express an involuntary or irresistible impulse. It often appears in the fixed phrases 不由自主 (bù yóu zì zhǔ, 'can't help oneself') and 不由得 (bù yóu de, 'cannot help but'). It indicates that an action or feeling arises spontaneously without conscious control. In sentences, it modifies a verb phrase to convey that someone does something involuntarily.