“Mm” or “Mmm” or “Mmmm” (“Mm! Mm! Good!” & “MMMM…TOASTY”)

“Mm!” is an expression of culinary delight. Campbell’s soups have been advertised as “M’m! M’m! Good!” since 1973. Quiznos sandwiches have been “MMMM…TOASTY” since that advertising began in 2002.
 
There are many “m” letters that can be added to the word. “U-m-m-m!” is cited in print from at least 1901 and is possibly the original word. The “mmm” sound is easy to make with the mouth closed (savouring the food?).
   
 
(Oxford English Dictionary)
mm, int. (and n.)
Forms: 19- m’m, 19- mm-m, 19- mmm, 19- mmmm, 19- mn (rare). Forms with m occurring five or more times are also occas. attested.  [Imitative. Compare H’M int., UM int.]

1. Expressing satisfaction, approval, or assent. Also (occas.) as n.
1911 E. FERBER Dawn O’Hara 185 Mm! What’s that smells so good, old girl?
1922 J. JOYCE Ulysses II. 54 You don’t want anything for breakfast? A sleepy soft grunt answered:—Mn.
1945 A. KOBER Parm Me 18 Mmm, is good the cake… You must use lotsa ecks [= eggs] fa such a cake.
1966 G. N. LEECH Eng. in Advertising xxii. 197 By tomorrow they’ll [sc. sandwiches] lose not a single ‘M’m’ of flavoursome moisture!
1974 D. FRANCIS Knock Down ii. 21 ‘You’d thought of that, had you?’.. ‘Mm,’ I said.
1987 Sunday Express Mag. 9 Aug. 20/1 Mm, I’m a bit embarrassed about that one because I was misquoted.
1995 A. WARNER Morvern Callar (1996) 160 You could get by with a Uh huh, a Mmm, giving the odd nod or coming out with a chuckle.
   
2. Expressing hesitation, reflection, or inarticulate interrogation. Cf. UM int.
1967 Listener 21 Sept. 367/2 Restraint is unnatural, mm?
1968 ‘P. HOBSON’ Titty’s Dead v. 66 She was mentioning..that she had had enough of policemen for the day. And, mm-m, that you could go chase yourself.
1983 R. CURTIS & R. ATKINSON in R. Curtis et al. Black-Adder (1998) 11/1 Percy. Mmmmmm? Yes, that’s the one, yes!
1995 K. O’RIORDAN Involved 170 Why don’t you just spit it out—say it right out—whatever it is that’s troubling you. Mmm?
1996 C. BATEMAN Of Wee Sweetie Mice & Men x. 76 Clay had enough clout..to insist on a mandatory defence of the heavyweight title despite the unforeseen (mmmmmmm) injury to Tyson’s scheduled opponent.
 
Google Books
Some Boys’ Doings
By John Habberton
Philadelphia, PA: George W. Jacobs & Co.
1901
Pg. 111:
“Wouldn’t it be splendid if one of old Mobdike’s biggest watermelons would pick itself and come floating across the river?”
 
“U-m-m-m!” responded the other boys.
   
March 1916, National Food Magazine, pg. 205:
Dandelion Greens: UM-M-M! Pass ‘Em To Me!
By WILLIAM HERSCHELL
(It’s a poem. From the last line—ed.)
Um-m-m! Um-m-! Pass ‘em to me!
“Songs of the Streets and Byways,” The Bobbs-Merill Co.
 
28 November 1917, Valdez (AK) Daily Prospector, pg. 1, cols. 3-4 photo caption:
“M-M-M!  Turkey for Thanksgiving!”
   
June 1935, Soda Fountain, pg. 23:
EVERYBODY GOES FOR
FROZEN MALTED . SOFT ICE CREAM (CREAMERY STYLE)
BIG SHAKE . FROZEN CUSTARD
UM-M-M-M!  UM-M-M!  UM-M!  UM-M!  UM-M!
 
21 April 1937, New York (NY) Herald Tribune, pg. 16, col. 8:
Check “Uhm-m, Good Gravy” in the coupon and send with a 3-cent stamp to the New York Herald Tribune Home Institute, 230 West Forty-first Street, New York City.
 
July 1944, International Steward, pg. 24, col. 2:
M—M—M—!
What beams of approval hot rolls or muffins bring…
   
December 1945, International Steward, pg. 15, col. 2 photo caption:
M-M-m-m!
 
(Trademark)
Word Mark M’M! M’M! GOOD!
Goods and Services IC 029. US 046. G & S: SOUPS. FIRST USE: 19731001. FIRST USE IN COMMERCE: 19731001
Mark Drawing Code (1) TYPED DRAWING
Serial Number 73553816
Filing Date August 16, 1985
Current Filing Basis 1A
Original Filing Basis 1A
Published for Opposition July 8, 1986
Registration Number 1411535
Registration Date September 30, 1986
Owner (REGISTRANT) CAMPBELL SOUP COMPANY CORPORATION NEW JERSEY CAMPBELL PLACE - P.O. BOX 391 CAMDEN NEW JERSEY 081010391
(LAST LISTED OWNER) CSC BRANDS LP LIMITED PARTNERSHIP DELAWARE CAMPBELL PLACE CAMDEN NEW JERSEY 081031799
Assignment Recorded ASSIGNMENT RECORDED
Attorney of Record Jennifer C. Debrow
Type of Mark TRADEMARK
Register PRINCIPAL
Affidavit Text SECT 8 (6-YR). SECTION 8(10-YR) 20060626.
Renewal 1ST RENEWAL 20060626
Live/Dead Indicator LIVE
 
(Trademark)
Word Mark MMMM…TOASTY
Goods and Services IC 030. US 046. G & S: sandwiches other than cracker sandwiches. FIRST USE: 20020415. FIRST USE IN COMMERCE: 20020415
Mark Drawing Code (1) TYPED DRAWING
Serial Number 76389519
Filing Date April 1, 2002
Current Filing Basis 1A
Original Filing Basis 1B
Published for Opposition October 7, 2003
Registration Number 2921118
Registration Date January 25, 2005
Owner (REGISTRANT) QUIZNO’S MASTER LLC, THE LTD LIAB CO COLORADO Suite 400 1475 Lawrence Street DENVER COLORADO 80202
Assignment Recorded ASSIGNMENT RECORDED
Attorney of Record Kristine M. Miller
Type of Mark TRADEMARK
Register PRINCIPAL
Live/Dead Indicator LIVE