see-saw

Concise Oxford English Dictionary © 2008 Oxford University Press:
see-saw

▶noun

• 1 a long plank balanced on a fixed support, on each end of which children sit and move up and down by pushing the ground alternately with their feet.

• 2 a situation characterized by rapid, repeated changes from one state or position to another.

▶verb change rapidly and repeatedly from one state or position to another and back again.

– origin C17 (orig. used by sawyers as a rhythmical refrain): reduplication of the verb saw1.