Isaiah 64:6 NASB "For all of us have become like one who is unclean, And all our righteous deeds are like a filthy (H5708) garment;
Strong's H5708
ʻêd, ayd; from an unused root meaning to set a period (compare H5710, H5749); the menstrual flux (as periodical); by implication (in plural) soiling:—filthy.
Online Dictionary of Biblical Usage:
I. menstruation
A. filthy rag, stained garment (fig. of best deeds of guilty people)
A.R. Fausset Commentary
6. unclean thing--legally unclean, as a leper. True of Israel, everywhere now cut off by unbelief and by God's judgments from the congregation of the saints.
righteousness--plural, "uncleanness" extended to
every particular act of theirs, even to their prayers and praises. True of the best doings of the unregenerate ( Phl 3:6-8 Tts 1:15 Hbr 11:6 ).
filthy rags--literally, a "menstruous rag" ( Lev 15:33 20:18 Lam 1:17 ).
fade. . . leaf-- ( Psa 90:5, 6 ).
So the point in Isaiah is not that our best isn't quite good enough. We are like 'unclean lepers' before a Holy God and the very BEST of our gifts to Him are like dirty, menstrual cloths. The modern equivalent of a soiled menstrual rag is a "used tampon".
Sort of puts things in perspective, don't it.