HAMLET: Thus conscience does make cowards of us all,
And thus the native hue of resolution
Is sicklied o’er with the pale cast of thought,
And enterprises of great pitch and moment
With this regard their currents turn awry
And lose the name of action.

William Shakespeare (1564-1616) English dramatist and poet

Hamlet, Act 3, sc. 1, l. 91ff (3.1.91-96) (c. 1600)
    (Source)

"Conscience" in this case is used in its archaic form, as consciousness, awareness.