Infirm of purpose
Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood.
I'll go no more.
I am afraid to think what I have done;
Look on't again I dare not.
Infirm of purpose!
Give me the daggers. The sleeping and the dead
Are but as pictures; 'tis the eye of childhood
That fears a painted devil.
As Macbeth returns from murdering King Duncan, Lady Macbeth
upbraids him for bringing back incriminating evidence. She thinks
that planting the murder weapon on the king's unconsious grooms and
smearing them with Duncan's blood will clear her and her husband of
any suspicion. This ploy doesn't work, but nevertheless she and
Macbeth are not immediately discovered.
When Lady Macbeth calls her husband "infirm of purpose," she
refers back to the root meaning of "infirm": unsteady, "not firm."
Macbeth's resolve ("purpose") is weak; he fears the deed he's done,
and thus he's also "infirm" in the modern sense: his will is
crippled. Shakespeare was the first to use the word "infirm" to
refer to physical infirmity—in the sense of weakness or disease—but
not in Macbeth. The heroine
of All's Well that Ends Well (1601) tells the sickly King of France
that she is able to heal him, and that "What is infirm from your
sound [healthy] parts shall fly" (Act 2, scene 1, 167).