Look at the verb "slow" in the Merriam-Webster Learner's Dictionary

1 a [no object] : to begin to move at a lower speed

  • The car slowed and gradually came to a stop.

— often + down or up

  • The car slowed down a little as it approached the intersection.
  • I slowed up and let the others catch up with me.

b [+ object] : to make (something, such as a car) move at a lower speed

  • The extra weight slowed the truck.

— often + down or up

  • Slow the car down so that I can read the sign.
  • The extra weight slowed up the truck.

My question is that why do we use "up" with the verb "slow"?

It makes more sense to use "down" with "slow" and "up" with "speed"

What does it mean by saying "I slowed my car up"? It sounds like "speed up".

For example, I slowed my car up and let the others catch up with me.

  • 4
    Americans may slow up where British speakers slow down. Both reduce speed. It's a regional difference. Jan 2 at 9:21
  • 3
    A question of the form "Why does (language X) say (Y) when it seems more logical to say (Z)?" almost always has as the complete and total answer "Because that's the way the people who speak X say it". Language is what people do, and has only a weak relationship with logic.
    – Colin Fine
    Jan 2 at 12:26
  • Can you cite specifically where you heard someone say "slow up"?
    – SegNerd
    Jan 3 at 22:52

"I slowed my car up" means that I slowed the speed of my car. Usually the phrase "slow up" will occur as an imperative.

Example: "We're coming up on the buoy." "Ok, slow up until you're right beside it."

As for why do (some) people use "slow up" instead of "slow down"? It depends on where the speaker is from. It is used in America, but it's certainly not uniform.

My experience has been that "slow up" is used in a more restrictive sense than "slow down" - "slow up" being used to gradually reduce your speed as you are approaching something.

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