What is the noun of fast?

The noun of "fast" depends on the context. Here are a few possibilities:

* Speed: If "fast" refers to a high speed, the noun could be speed, velocity, or rapidity.

* Fasting: If "fast" refers to abstaining from food, the noun is fast.

* Fastener: If "fast" refers to a device that secures something, the noun is fastener.

To determine the correct noun, you need to consider the sentence and the meaning of "fast" within it.

For example:

* "The car drove at a speed of 100 mph." (speed)

* "She is on a fast to lose weight." (fast)

* "The fastener broke on my backpack." (fastener)