Why does the Litany of Saint Joseph call him "terror of demons"? A litany for Joseph, the husband of Mary, was approved by Pope St Pius X in 1909 (*). Most of the invocations are straightforward to understand, referring to his life (eg. &quot;husband of the Mother of God&quot;, Dei Genetrice sponse), virtues (&quot;most faithful&quot;, fidelissime), and traditional associations/patronage (&quot;model of artisans&quot;, exemplar opificium). There is one bit which is surprising to me -

Hope of the sick, pray for us. Patron of the dying, pray for us. Terror of demons, pray for us. Protector of the Holy Church, pray for us.
Spes aegrotantium, ora pro nobis. Patrone morientium, ora pro nobis. Terror daemonum, ora pro nobis. Protector sanctae Ecclesiae, ora pro nobis.

I was not aware that St Joseph had anything in particular to do with demons. I can see the part about the sick and dying as an extension of his general image as a protector, especially in relation to the family. Is the demon reference the same sort of thing? Or is there a specific reason - perhaps a traditional story of some kind - why Joseph is called the terror of demons?
* Urbis et orbis: adprobantur litaniae in honorem S. Joseph sponsi B. Mariae V., eisque adnectitur indulgentia. Sacred Congregation of Rites, 18 March 1909. In Acta Apostolicae Sedis 1(6):290-292, 1909. OCR'd version with some errors.
St. Joseph is the terror of Demons because all of demons effort is nullified if one seeks refuge to St. Joseph  and also because he is pure, and demons cannot stand against his purity.

Even though there is no official dogmatic pronunciation in this regard by the Catholic church, numerous private revelationsexample and traditions have held that St. Joseph, after his conception, was, through the future merits of Jesus cleansed from the stain of original sin. And also Catholic tradition holds that St. Joseph did not sin all thru his life.ref Satan and his cohort of fallen angels had no success in seducing Joseph. These are not dogmas however and Catholics are not obliged to believe these traditions.

Catholic Online says about why St.Joseph is the Terror of demons:


  Those who turn to Saint Joseph for his powerful intercession and good example do much to keep Satan and the other wholly corrupt devils at bay. In this sense, Joseph is the Terror of Demons because when the friends of Jesus avail themselves of his salutary influence, Saint Joseph is for them a sure protection against the oppressive wiles of the Prince of Darkness. src


One more POV in this regard:


  But evil shrinks from Joseph for He is pure. ... The demons can't "see" Joseph because he is pure. Purity makes him invisible, just as pure water is perfectly clear. Evil spirits don't know when, how, or from which direction Joseph is going to hammer them. Joseph's job, when Christ was a child and up to this very day, is to protect the Holy Family, and through our baptisms, we are part of his family. src