Three score and ten
The span of a life. In the days that this was coined that was considered to be seventy years.
Threescore used to be used for sixty, in the way that we still use a dozen for twelve, and (occasionally) score for twenty. It has long since died out in that usage but is still remembered in this phrase. Threescore goes back to at least 1388, as in this from John Wyclif's Bible, Leviticus 12, at that date:
"Thre scoor and sixe daies."
There are numerous uses of 'threescore' in the Bible. Most of them refer to its simple meaning as the number sixty, for example:
"...threescore and ten bullocks, an hundred rams, and two hundred lambs: all these were for a burnt offering to the Lord."
There is a use of it that refers to the span of our lives, in Psalms 90:
The days of our years are threescore years and ten;
and if by reason of strength they be fourscore years,
yet is their strength labor and sorrow;
for it is soon cut off, and we fly away.
As with many other Biblical phrases, this was picked up by Shakespeare. In Macbeth, 1605, we have:
Threescore and ten I can remember well:
Within the volume of which time I have seen
Hours dreadful and things strange; but this sore night
Hath trifled former knowings.
It's an odd fact that, although Shakespeare took numerous phrases and examples of imagery from the Bible, the word Bible doesn't appear in any of his plays.
See other phrases and sayings from Shakespeare.