A dense thicket or area overgrown with ferns, forming a brake or impenetrable tangle of vegetation.
Compound of 'fern' and 'brake' (an old word for brushwood or dense vegetation). Medieval English used 'brake' to describe any thorny or bushy area.
Medieval hunters used the word 'brake' for any place so dense you'd break your legs trying to run through it—fernbrakes were particularly treacherous!
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