IPA: /ˈræpɪdnəs/
KK: /ˈræpɪdnəs/
The quality or state of being quick or fast.
The rapidness of the river's current made it difficult to swim.
Rapidness → It is formed from "rapid" (from Latin "rapidus", meaning swift or quick) and "-ness" (meaning the state or quality of). The word "rapidness" refers to the quality of being swift or quick.
Think of 'rapid' meaning swift, and remember that 'rapidness' describes the state of being swift.