IPA: /ˈɒnˌlʊkə/
KK: /ˈɑnˌlʊkər/
A person who watches something happening, especially an event or activity, without being involved in it.
The onlooker watched the street performance with great interest.
Onlooker → It is formed from "on" (meaning in a position of) and "look" (from Old English "locian", meaning to see or observe) and "-er" (meaning a person who). An onlooker is a person who is in a position of observing or watching something.
Think of someone being 'in a position of' ('on') 'seeing' ('look') — that's why an onlooker is someone who watches.