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.
Lady Macbeth:Macbeth Act 2, scene 2, 45–52
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).