I've taken the liberty to consolidate and alphabetize all the above. Perhaps this could replace this thread and/or be given a more prominent position? (While keeping open the option for people to ask questions as they come up.)

AR - Antiphonale Romanum
AUG - Anglican Use Gradual
BFW = By Flowing Waters, Paul Ford's English-language adaptation of the Graduale Simplex
CCW = Corpus Christi Watershed, non-profit foundation publishing sacred music
CDWDS or CDW - Congregation for Divine Worship (and Discipline of the Sacraments). The office in Rome charged with regulating the liturgy for the whole Latin Church.
CMAA - Church Music Association of America
CN - Canticum Novum (Fr Anthony Ruff's new book of chant)
EF - Extraordinary Form (ie, the TLM, or "Latin Mass")
F: form.
FACP - Full, active, and conscious participation; a goal explicitly stated by many progressive liturgists, and implicitly desired by many traditionalists. The exact meaning/interpretation is a source of continued debate, the crux of the matter having to do with both a matter of translation ("active" vs. "actual") and a matter of temperament (the question of what, precisely, constitutes "participation" in a Liturgical action).
G&P - Glory & Praise (either the hymnal by that name, or the style in general)
GABC - input notation for the chant typesetting program Gregorio: see gregoriochant.org for info
GIA- (has nothing to do with the words the initials stand for.) Large music publisher
GIRM - General Instruction of the Roman Missal
GM - Gregorian Missal (version of the Graduale Romanum but with English translations)
GN: Graduale Novum (edition of Graduale with melodies revised according to research in recent decades)
GR - Graduale Romanum, the normative latin music book for the latin rite, mostly containing melismatic propers, but also the ordinary chants of the Mass and other music.
GS - Graduale Simplex, the other normative latin music book for the latin rite, contains simplified antiphons (often seasonal in usage) for usage in the Mass for when the GR propers are too difficult.
GT - Graduale Triplex
IATBOL - "I Am The Bread Of Life" by Sr. Toolan; however, you're not the Bread of Life: He is.
ICEL - International Commission on English in the Liturgy
iij = sing three times
IIRC = "if I remember correctly"
IMHO / IMNSHO - In my humble opinion / in my not so humble opinion
ITT = "In this thread"
LC - Liber Cantualis
LH - Liber Hymnarius
LOTH - Liturgy of the Hours
LU - Liber Usualis
MC - Master of Ceremonies
MCW - Music in Catholic Worship (1980s US Bishops document that was replaced by STTL)
MP - motu proprio
MSF - Musica Sacra Forum
MS = Musicam sacram, 1967 document
NAB - New American Bible (translation of the Bible currently in the US lectionary)
NLM - New Liturgical Movement
NPM = another group for Catholic musicians
NRSV - New Revised Standard Version (of the Bible)
OCP- Oregon Catholic Press- Large Catholic music publisher
OEW = "On Eagle's Wings", sometimes snarkily called the "Yoo-hoo song" from its first words
OF - Ordinary Form (ie, the "New Mass," or Novus Ordo)
OP - The Original Poster (person who started the thread)
OT - Offertoriale Triplex
OT - Ordinary Time
P&W - Praise & Worship
PBC - Parish Book of Chant
PBEH = the project "Parish Book of English Hymns" (see http://musicasacra.com/english-hymns-in-the-commons/ )
PF: previous form.
PG - Plainchant Gradual (or P&B - Palmer and Burgess)
PIP - People in the Pews. That is, regular churchgoers.
PP - a previous poster (or the previous poster) in the thread
PTB - Pray Tell Blog
RC - Roman Catholic
RGP - Revised Grail Psalter
ROTR - Reform of the Reform (as in, undoing the bad things that happened as misinterpretations of Vatican II)
RSV - Revised Standard Version (of the Bible)
SC - Sacramentum Caritatis
SC - Sacrosanctum Concilium (Vatican II document on the Sacred Liturgy)
SDPP - St Dunstan's Plainsong Psalter
SEP - Simple English Propers, a book of english propers by Adam Bartlett for the OF
SLJ - St. Louis Jesuits
SP - Summorum Pontificum
STTL - Sing to the Lord (US Bishop document on music in the liturgy)
TLM - Traditional Latin Mass (or possibly, "Tridentine Latin Mass")
USCCB - United States Conference of Catholic Bishops
V2 - Vatican II (Second Vatican Council)
VII (Vii, V II)- Vatican II (Second Vatican Council)
WLP- World Library Publications- Large music publisher
YMMV - Your mileage may vary

Carol,

Except in Europe the following are standard abbreviations:

FSSP = Priestly fraternity of St. Peter, a 1988 off-shoot of the Priestly Fraternity of Pope St. Pius X

SSPX (sometimes FSSPX) the group of priests gathered together originally under the auspices of Archbishop Marcel Lefebvre, forming the Priestly Fraternity of St. Pius X.

ICK, ICRSS, ICKSP, -- The Institute of Christ the King, Sovereign Priest.

I think it is interesting that there is enough agreement/discussion about certain hymns that they have earned abbreviation status. A hymn must have earned "a reputation" to achieve abbreviation

"WPI" - Please translate. Thanks.

It took me a while to figure that out. I was confused out the leadership of the Worcester Polytechnic Institute got embroiled in Catholic traditionalist agitprop. Another God, another hilltop? I wondered (WPI and the College of The Holy Cross are on hills about 2.5 miles apart.)

OCM - Ordo Cantus Missae