It is very informal, so only say it humorously to friends. 'There are no flies on you' means that 'you are very eager to accomplish' whatever task is under discussion, 'you are quick to act' in taking advantage of a situation. ' The image of the idiom is an animal which does not allow flying insects to alight on its body, by switching its tail or otherwise remaining very active.
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01cite10Anonymous12cite10Hi There 12br10I've never he
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10It's used to imply that someone is not 'the sharpest tool in the shed.' It is used sarcastically when someone says something stupid or is a bit slow to catch on... 12br
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10Hope this helps12br
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01cite10Anonymous12cite10It's used to imply that someone is not 'the sharpest tool in the shed.' It is used sarcastically when someone says something stupid or is a bit slow to catch on...12blockquote10 I use it to mean exactly the opposite.02br