A young person, usually a child or teenager.
From 'young' plus the agent suffix '-ster' (one who is). This pattern created many English words like 'gangster' and 'punster' to describe someone as having a quality.
English loves adding '-ster' to words to make them mean someone associated with that thing: a young person becomes a 'youngster,' just like a jester tells jokes and a punster makes puns.
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