In John Webster's play, The Duchess of Malfi, Antonio says of the Duke:
He never pays debts unless they be shrewd turns,
And those he will confess that he doth owe.
I really don't understand what he means. Can someone explain?
I haven't found a reliable source to corroborate, but this set of annotations has a likely interpretation:
shrewd turns: he only repays debts to those who have wronged him, i.e. revenge.
This usage comes from a sense of shrewd that was already obsolete in 1828:
Painful; vexatious; troublesome.
Every of this number / That have endured shrewd nights and days with us. Obs. Shak.
No enemy is so despicable but he may do one a shrewd turn. Obs. L'Estrange.
Another usage, from Freemasons' proceedings:
Consider also whether you may not come hereafter to be acquainted with him, related to him, or in want of his favor, whom you have thus injured, and whether it may not be in his power to revenge a spiteful and needless word by a shrewd turn.
And another in a passage about turning the other cheek:
... rather than revenge thyself for one injury, receive another: and rather than vex him who forces thee to go a mile, go with him two miles: not that Christ intends thou shouldst offer to do thyself a shrewd turn, or invite another; nor that thou shouldst suffer it, if thou canst fairly avoid it....
Thus, a shrewd turn is a painful or vexatious deed: injury or insult.