To disturb or drive away from a roost; to cause birds to leave their roosting place.
From dis- + roost (from Old English hrost). The prefix dis- adds the sense of disturbance or removal, making it mean to drive from a roost.
Farmers and hunters use 'disroost' to describe flushing birds out of trees or barns. It's a wonderfully specific word for a very specific agricultural task that predates industrial farming.
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