subtile > rarefied, thin; subtle
4 Like a young squire, in loves and lustihead
lustihead > libidinousness, pleasure; lustfulness
5 His wanton days that ever loosely led,
that > (Relative to "Squire" in line 4, i.e. "Like a young squire, that in loves and lustihood His wanton days ever loosely led)
6 Without regard of arms and dreaded fight:
arms > deeds of arms
7 Those two he took, and in a secret bed, 8 Covered with darkness and misdeeming night,
misdeeming > {Misleading; to "misdeem" is also to misjudge unfavourably, to deem evil}
9 Them both together laid, to joy in vain delight.