windhover
A kestrel, which is a small falcon that hovers (thus wind-hover) with rapidly beating wings while searching for prey on the ground
minion
In modern usage, a follower or underling of a powerful person, especially a servile or unimportant one. But Hopkins likely has in mind one of the word’s older senses, i.e., one highly favored by.
dauphin
Literally, French for “dolphin”; formerly, and more relevant here, “dauphin” referred to the eldest son of the King of France and heir-apparent to the throne
dapple
A patch or spot of color or light
wimpling
A wimple is the cloth headdress worn by nuns; here, as a verb, the relevant meanings are: to cover with or as if with a wimple; to cause to ripple.
rebuffed
To reject or criticize sharply
buckle
Bend and give way under a weight or force
chevalier
A knight or chivalrous/distinguished man; here, roughly, "champion"
sillion
The thick, voluminous, and shiny soil turned over by a plow.
gall
to fret and wear away by friction
vermilion
a brilliant red color
