(yōʹyō´) noun. plural yo-yos. 1. A toy consisting of a flattened spool wound with string that is spun down from and reeled up to the hand by motions of the wrist. 2. Informal. One that undergoes frequent abrupt shifts or reversals, as of opinion or emotion; a vacillator. 3. Slang. A person regarded as stupid or objectionable, yo-yo.