What does it mean to be "poor in spirit"? I'm working through a study on "The Beatitudes" of Matthew 5 and would appreciate any insight into Christ's statement:


  Blessed are the poor in spirit, for theirs is the kingdom of heaven.
  ESV


My question is specifically in regard to what "poor in spirit" means, not "kingdom of heaven."

An interesting paradigm shift came to me from Dallas Willard's The Divine Conspiracy, 1997, which inverts the apparent formulaic assumption of "poor in spirit" to represent a bad thing. on pg. 99 of his aforementioned book he puts it this way:


  [Willard's paraphrase of Matt. 5:3] "Blessed are the spiritual zeros -
  the spiritually bankrupt, deprived and deficient, the spiritual
  beggars, those without a wisp of 'religion' - when the kingdom of the
  heavens comes upon them."
  ...
  
  ... The poor in spirit are blessed as a result
  of the kingdom of God being available to them in their spiritual
  poverty.


So, seeing "poor in spirit" turned into a bad thing that the kingdom of heaven's availability turns into a good thing was an interesting idea to me, but Williard fails to provide much objective or external support for this beyond his own reasoning. If you're aware of an objective or external argument supporting this perspective, please share it.

In short, any answers that can expound on the meaning of "poor in spirit" would be greatly appreciated. Please avoid giving me opinions, and please provide references for all answers, if you can. Thanks!
The Old Testament and Hebrew linguistic background of this passage should be kept in mind. Recall:   


Matthew (or his source) was likely translating words that were spoken in Hebrew or Aramaic.  
Both Jesus and Matthew likely had a Semitic mother tongue.   
The Hebrew bible (both in its Hebrew and Greek forms) was frequently referenced explicitly by both, and in many more cases provided unstated background.


The term used is οἱ πτωχοὶ τῷ πνεύματι, (hoi ptōchoi tō pneumati) &ndash; "the poor in [or with respect to] spirit". The dative translated "in" has a broad range of meanings; this needs to be determined from context. A few relevant points of context from the Hebrew scriptures: 


The Psalms, regarding poor. 

The word ptōchos is the preferred (thought not exclusive) translation of the Hebrew עני (ʿānî) which carries the basic idea of "lacking" &ndash; at times in the sociologic sense (lacking property), but more often in the Psalms it denotes "neediness" with reference to God. See especially Psalms 10 and 72, where it is also sometimes translated "afflicted". As far as I see, in all of its 31 appearances in the Psalms ʿānî is used in a positive sense, characterizing those who recognize their need of God. 1 
Psalm 51:17,* regarding spirit.


  The sacrifices of God are a broken spirit;
  &nbsp;&nbsp;&nbsp;a broken and contrite heart, O God, you will not despise.


The "spirit" here is pneuma in the Greek Septuagint, the same word used in Matthew 5. This provides a background for the idea that having a pneuma that is "compromised" in some way is a rhetorical indication of humility toward God.
Isaiah 66:2, regarding poor in spirit.


  All these things my hand has made,
      &nbsp;&nbsp;&nbsp;and so all these things came to be,
      &nbsp;&nbsp;&nbsp;declares the LORD.
   But this is the one to whom I will look:
      &nbsp;&nbsp;&nbsp;he who is humble and contrite in spirit
      &nbsp;&nbsp;&nbsp;and trembles at my word.   


The word that the ESV translates "humble" is ʿānî (see #1, above), and "spirit" is translating ruach, the normal Hebrew correlate of pneuma. Here "humble and contrite in spirit" is helpfully restated: "trembles at my word." 


 
*All quotes are ESV.





Note


1.   In this way, Luke’s "poor" that parallels Matthew’s "poor in spirit" (Luke 6:20 // Mat. 5:3) may carry the same sense (or even translate the same Semitic term?), despite the fact that the Greek ptōchos itself more often refers to economic disadvantage.
 


References


R. T. France, The Gospel of Matthew (NICNT; Eerdmans, 2007).
  


Ludwig Koehler, Walter Baumgartner, et al., The Hebrew and Aramaic Lexicon of the Old Testament (Leiden; New York: E.J. Brill, 1999).
  


William Arndt, Frederick W. Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature, 3rd ed. (Chicago: University of Chicago Press, 2000).
  


The links in the body of the answer are to older lexicons, provided for convenience because they are freely available. It is the newer lexicons cited here that were used to inform this answer.