"Sin boldly" or "Let your sin be strong" (depending on translation) in context.  This was taken from blog:
http://www.holytrinitynewrochelle.org/yourti19047.html  Did Luther really say, "Sin boldly!" Yes, but one cannot understand what he was saying at all without the rest of the sentence "...but believe more bolder still." To see what he was speaking about we need to look at the letter from which these bold words are lifted. He wrote in the translation we have here, "God does not save those who are only imaginary sinners.  Be a sinner, and let your sins be strong, but let     your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world."   He wrote this to his colleague Philipp Melachthon from his hiding place, the Wartburg Castle, in 1521.
Read the whole context in:
                     A Letter From Luther to Melanchthon
      Letter no. 99, 1 August 1521, From the Wartburg
                                  (Segment)
                                Translated by
                            Erika Bullmann Flores
   from: _Dr. Martin Luther's Saemmtliche Schriften_
                        Dr, Johannes Georg Walch, Ed.
        (St. Louis: Concordia Publishing House, N.D.),
                          Vol. 15,cols. 2585-2590.                                 
    Of course, you can only know and absolve those sins which have been
    confessed to you; sins which have not been confessed to you, you 
    neither need to know nor can you absolve them. That is reaching too
    high, dear gentlemen."   
    You cannot convince me that the same is true for the vows made by
    priests and monks.  For I am very concerned about the fact that the
    order of priesthood was instituted by God as a free one. Not so that
    of the monks who chose their position voluntarily, even though I have
    almost come to the conclusion that those who have entered into that
    state at an age prior to their manhood, or are currently at that
    stage, may secede with a clear conscience. I am hesitant, however,
    with a judgment about those who have been in this state for a long
    time and have grown old in it.   
    2. By the way, St. Paul very freely speaks about the priests (1.Tim:
    4, ff), that devils have forbidden them to marry; and St. Paul's
    voice is the voice of the divine majesty. Therefore, I do not doubt
    that they must depend on him to such a degree that even though they
    agreed to this interdiction of the devil at the time, now--having
    realized with whom they made their contract--they can cheerfully
    break this contract.   
    3. This interdiction by the devil, which is clearly shown by God's
    Word, urges and compels me to sanction the actions of the Bishop of
    Kemberg. For God does not lie nor deceive when He says that this is
    an interdiction from the devil.  If a contract has been made with the
    devil it must not endure since it was made in godless error against
    God and was damned and repudiated by God.  For He says very clearly
    (1. Tim. 4:1 Vulg.) that those spirits are in error who are the
    originators of the interdictions.    
    4. Why do you hesitate to join this divine judgment against the gates
    of hell? That is not how it was with the oath of the children of
    Israel which they gave to the Gibeons.  They had it in their laws
    that they must offer peace or accept peace offered to them, and
    accept into their midst proselytes and those who adhered to their
    customs.  All this took place. Nothing happened there against the
    Lord or by the advice of spirits. For even though in the beginning
    they murmured, later on they approved.   
continued next post...