Tomboyism involves not courting male attention in the obvious ways, but, rather, assuming it, knowing it's there and doing whatever you want: It telegraphs independence, seriousness and freedom.
WSJ: Girls Will Be Boys | Fashion
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There is also the idea in tomboyism that you have chosen comfort over flirtation, ease over fussiness, a button-down shirt you've grabbed off the floor over a skirt you can't sit on the grass in.