A person who is faint or faints easily; a weak or cowardly person.
From 'faint' plus the diminutive or agent suffix '-ling.' The suffix '-ling' (as in 'duckling' or 'weakling') creates nouns for things characterized by the root quality, often with a somewhat diminutive or derogatory tone.
The '-ling' suffix has an interesting double quality: it can be cute (like 'duckling') or insulting (like 'weakling')—so 'faintling' would be a mildly disparaging term for someone prone to fainting, used with a mix of sympathy and scorn.
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