seesaw

What it means

1. A structure composed of a plank, balanced in the middle, used as a game in which one person goes up as the other goes down.

   Synonyms: teeter-totter

2. The game or pastime of seesawing.

   Now Jill did laugh And Jack did cry, But his tears did soon abate, Then Jill did say, That they should play, At sea-saw across the gate.

   1810, Jack and Jill, and old Dame Gill, York: J. Kendrew, page 8

3. figurativelyA series of up-and-down movements.

In a sentence

Now Jill did laugh And Jack did cry, But his tears did soon abate, Then Jill did say, That they should play, At sea-saw across the gate.

1810, Jack and Jill, and old Dame Gill, York: J. Kendrew, page 8

Where it comes from

Probably a frequentative imitative of rhythmic back-and-forth, up-and-down or zigzagging motion, such as teeter-totter, zigzag, flip-flop, ping pong, etc., under the umbrella term of reduplication; also likely influenced by the verbs see and saw of either present or past tense.