To molest someone is to annoy or attack them. Although the word was first used to mean "pester," now it is more typically used to refer to criminal abuse.
The verb molest can be used to talk about any kind of persistent harassment, but it most commonly describes an unwanted or aggressive sexual attack. To molest someone in this in way is a very serious criminal assault. The word originally meant simply "to cause trouble or grief," and it comes from the Latin word molestare, "to disturb, trouble, or annoy." It wasn't used to mean "attack sexually" until about 1950.