1. something that fluctuates, moves up and down repeatedly
go up and down like a yo-yo
A Yo-Yo is a children's toy that is made to be bounced up and down. Something that is 'like a Yo-Yo' behaves, in the same way, fluctuating moving constantly unable to maintain a steady level rate or speed. This phrase is a simile. "My dog is like a Yo-Yo, always jumping up and down." "You need to be aware when you make an investment that interest rates are like a Yo-Yo, they can change many times over the course of a few years."