IPA: //bɪˈsɛtə//
KK: /bɪˈsɛtər/
A person or thing that troubles or harasses someone or something.
The besetter of the peaceful town caused much distress among the residents.
Besetter is formed from "be-" (meaning about or around) and "setter" (from Old English "settan", meaning to set or place). The word describes someone who sets about or surrounds something, often in a troublesome or persistent manner.
Think of someone who is 'setting' ('setter') something 'around' ('be-') — that's why a besetter is someone who persistently surrounds or troubles another.