Screw your courage to the sticking place
Macbeth:
If we should fail? Lady Macbeth:
We fail?
But screw your courage to the sticking place,
And we'll not fail.
This is one time when you don't have to feel ignorant because you don't understand what Shakespeare really meant. Although he invented "sticking place," and though our usage derives directly from this scene, Shakespeare never explains what the phrase means.
Macbeth still has cold feet; he and his wife have agreed to kill King Duncan of Scotland, but he can't stop thinking of all the consequences the deed might not trammel up [see THE BE-ALL AND THE END-ALL]. Lady Macbeth, after impugning her husband's manliness, urges him, as we might say, to "screw up his courage." The OED suggests that Lady Macbeth's original words refer to the twisting of a tuning peg until it becomes set in its hole. The editor of The Riverside Shakespeare, on the other hand, suggests that a "sticking place" is "the mark to which a soldier screwed up the cord of a crossbow." Whether the metaphor is musical, martial, or otherwise, Lady Macbeth's meaning is obvious though her words are obscure: "tighten up your courage until it is fixed in the place necessary for the murder of Duncan."