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At roost Explained

At roost at English => English (Websters 1913) Of Explained:

Roost \Roost\, n. [AS. hr[=o]st; akin to OD. roest roost,
roesten to roost, and probably to E. roof. Cf. {Roof}.]
1. The pole or other support on which fowls rest at night; a
perch.

He clapped his wings upon his roost. --Dryden.

2. A collection of fowls roosting together.

{At roost}, on a perch or roost; hence, retired to rest.