The 'monastic' last psalms of Vespers - Ps 140 on Thursdays

Psalm 140 can be interpreted several ways including as referring to the Passion (the evening sacrifice) and institution of the New covenant; the Office as a substitute for the Temple sacrifices; on the need to resist pressure to conform, but instead stay on the path of righteousness; and more.

All of these are clearly relevant to monks.

The verse I want to suggest has a particular importance though, is verse 3, set a watch before my mouth. 

Vulgate	Douay-Rheims
Psalmus David.	A psalm of David.
1 Dómine, clamávi ad te, exáudi me: * inténde voci meæ, cum clamávero ad te.	I have cried to you, O Lord, hear me: hearken to my voice, when I cry to you.
2  Dirigátur orátio mea sicut incénsum in conspéctu tuo: * elevátio mánuum meárum sacrifícium vespertínum.	2 Let my prayer be directed as incense in your sight; the lifting up of my hands, as evening sacrifice.
3  Pone, Dómine, custódiam ori meo: * et óstium circumstántiæ lábiis meis.	Set a watch, O Lord, before my mouth: and a door round about my lips.
4  Non declínes cor meum in verba malítiæ: * ad excusándas excusatiónes in peccátis.	4 Incline not my heart to evil words; to make excuses in sins.
5  Cum homínibus operántibus iniquitátem: * et non communicábo cum eléctis eórum	With men that work iniquity: and I will not communicate with the choicest of them
6  Corrípiet me justus in misericórdia, et increpábit me: * óleum autem peccatóris non impínguet caput meum.	5 The just man shall correct me in mercy, and shall reprove me: but let not the oil of the sinner fatten my head.
7  Quóniam adhuc et orátio mea in beneplácitis eórum: * absórpti sunt juncti petræ júdices eórum.	For my prayer shall still be against the things with which they are well pleased: 6 Their judges falling upon the rock have been swallowed up.
8  Audient verba mea quóniam potuérunt: * sicut crassitúdo terræ erúpta est super terram.	They shall hear my words, for they have prevailed: 7 As when the thickness of the earth is broken up upon the ground:
9  Dissipáta sunt ossa nostra secus inférnum: * quia ad te, Dómine, Dómine, óculi mei: in te sperávi, non áuferas ánimam meam.	Our bones are scattered by the side of hell. 8 But to you, O Lord, Lord, are my eyes: in you have I put my trust, take not away my soul.
10  Custódi me a láqueo, quem statuérunt mihi: * et a scándalis operántium iniquitátem.	9 Keep me from the snare, which they have laid for me, and from the stumbling blocks of them that work iniquity.
11  Cadent in retiáculo ejus peccatóres: * singuláriter sum ego donec tránseam.	10 The wicked shall fall in his net: I am alone until I pass.

 St Benedict on keeping silence

St Benedict refers to the importance of control over speech in many places in his Rule, but key summaries of his teaching on the subject are contained in chapter 4 the tools of good work), 6 (On silence) and 7 (where he devotes three of the twelve steps of humility to the topic).

In these chapters St Benedict built on an already well established monastic tradition that taught that speech too often and easily constituted sin, but even where it did not, it could be an impediment to the receptive listening necessary to progress in the spiritual life. 

Both Chapters 4 and 6 include commentary that uses words that echo Psalm 140.  Chapter 4 contains a series of injunctions on the subject of control over what is said including 'Guard your lips from harmful or deceptive speech' (os suum a malo vel pravo eloquio custodire).  And chapter 6 opens with a quote from Psalm 38:

Let us do as saith the prophet: I said, I will take heed unto my ways, that I offend not with my tongue. I have set a guard to my mouth [posui ori meo custodiam]. I was dumb and was humbled, and kept silence even from good words.

The key distinction between Psalm 38's text and Psalm 140 goes to the role of grace: in Psalm 38 the speaker has made a deliberate decision not to speak, lest he stir up those who inevitably attack the good; in Psalm 140 we ask for God's help in withstanding such assaults.

Compline and the Great Silence

Presumably in part because the verse corresponds well to the verse that opens each day in the monastic life, O Lord open my lips that I may announce your praise, a seventh century document describing Roman Benedictine practices (Ordo XVI) instructs it to be used as the last thing said each night after Compline, to mark the start of the Great Silence. 

St Jerome's commentary on the psalm includes a reference to the verse now used to open Compline, perhaps helping to explain the reason for its selection:

'Death and life are in the power of the tongue'; and again: 'I tell you, that of every idle word men speak, they shall give account on the day of judgment.' The prophet prays, therefore, that his words may not be vain, but holy and pleasing to God. 'A guard at the door of my lips.' He is asking for a guard round about his lips like the rampart of a castle, that he may never capitulate to sin...The prophet prays earnestly, therefore, that a sentry be placed around all his senses, that his whole household be fortified against the invasion and conquest of his adversary the devil. St. Peter writes: 'Our adversary the devil, as a roaring lion, goes about seeking someone to devour.' Not ordinarily does the devil make his attack through grave faults but through slight ones, that in some way or other he may gain admittance, win his victory, and ultimately impel his man to greater vices. Not through fornication or avarice, but through lesser sins, he secures an entrance.