What is the meaning of the word "bob"?

Definitions:

a short abrupt inclination, usually of the head noun
- I gave them a short bob of acknowledgement.
to cut hair in the style of a bob verb
- I'm going to ask my hair stylists if they can bob my hair.
to make a curtsy verb
- I bobbed before the host.
to move up and down repeatedly verb
- My rucksack bobbed gently on my back as I traipsed through the forest unevenly.
to remove or shorten the tail of an animal verb
- The breeder bobbed the dog's tail.
to ride a bobsled verb
- They bobbed down the hill screaming with pleasure.