From Middle English boskage, forest, probably of Germanic origin.
'Bosk' and 'bosket' have the same definition as boskage.
Usage is mostly literary or poetic (maybe that's why the Knights of Ni didn't demand a boskage).
Outside, in the garden, it was playtime. Naked in the warm June sunshine, six or seven hundred little boys and girls were running with shrill yells over the lawns, or playing ball games, or squatting silently in twos and threes among the flowering shrubs. The roses were in bloom, two nightingales soliloquized in the boskage, a cuckoo was just going out of tune in the lime trees. The air was drowsy with the murmur of bees and helicopters.
- Aldous Huxley, "Brave New World"
'Bosky' is a poetic term for 'shady'; the word origin is listed as Middle English bosk, bush.
And with each end of thy blue bow dost crown
My bosky acres and my unshrubb'd down,
Rich scarf to my proud earth; why hath thy queen
Summon'd me hither, to this short-grass'd green?
- Shakespeare, "The Tempest"
(Not trying to step on uniquepov's territory: I don't think this is a Shakespeare-coined word.)