A young person who is learning to flirt or who flirts in an inexperienced, endearing way.
From 'flirt' plus the diminutive suffix '-ling' (from Old English 'ling'), which suggests something young, small, or less important, as seen in 'duckling' and 'underling'.
The '-ling' suffix has been making cute or diminutive nouns for over a thousand years in English—it's the same suffix that gave us 'gosling' (young goose) and 'princeling' (minor prince), showing how this pattern lets us playfully categorize inexperienced or smaller versions of things.
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