to move gently and repeatedly up and down, typically on the surface of liquid or into and out of view

His eyes fixed on a cork that bobbed aimlessly on the water — Kate Chopin

to curtsy briefly

(+ for) to try to seize a suspended or floating apple, etc with the teeth

bobbing for apples at Halloween

to move (one's head, etc) up and down in a short quick movement

a short quick down-and-up motion

a change of order in bell-ringing

a short straight hairstyle for women, with the hair cut evenly at about chin level, so that it swings loosely

= float¹ (1a)

a hanging ball or weight on a plumb line or kite's tail

a knot or twist, e.g. of ribbons or hair

Scot a nosegay

to cut (something) shorter; esp to crop (a horse's tail)

to cut (hair) in a bob