Here's from Hamlet, Act 1, Scene 2 (The Arden Shakeseare edited by Harold Jenkins)
Fie, 'tis a fault to heaven,
A fault against the dead, a fault to nature,
To reason most absurd, whose common theme
Is death of fathers, and who still hath cried
From the first corse till he that died today,
'This must be so'.
I understand the grammar structure of this sentence except "To reason most absurd". I guess it means "it is irrational", but I can't analyze it grammatically.