The phrase the dog ate my homework and variants are used as, or denote, an unconvincing or far-fetched excuse:
– for failing to hand in school homework,
and, by extension:
– for any failure to do or produce what was expected.

The earliest mention that I have found of a person blaming a dog for their own unpreparedness is from More Memories: Being Thoughts about England spoken in America (London: Edward Arnold, 1894), by the English Anglican priest Samuel Reynolds Hole (1819-1904):

There is one adjunct of a sermon, which nearly all who hear admire, and which all who preach may possess if they please—brevity. Unhappily, the speakers, whom this virtue would most gracefully become, do not seem to be aware of its existence; like Nelson, they put the telescope to the blind eye, when signals are made to “cease firing.” They decline to notice manifest indications of weariness, yawns, sighs, readjustment of limbs, ostentatious inspection of watches; and they seem rather to be soothed than offended by soft sounds of slumber, as though it were music from La Somnambula.
One of these tedious preachers went away for his holiday, and the clergyman who took his duties in his absence apologized one Sunday to the clerk in the vestry, when the service was over, for the shortness of his sermon: a dog had been in his study, and torn out some of the pages. “Oh, sir,” said the clerk, a bright beam of hope on his countenance, “do you think that you could spare our vicar a pup?”

This story has often been repeated, and elaborated on, since 1894. For example, the following version is from the President’s Address, in the Proceedings of the Forty-second Annual Meeting of the Fire Underwriters’ Association of the Northwest Held at the Hotel La Salle, Chicago, Illinois, October 4^th and 5^th, 1911 (Printed by order of the Association, 1911):

In my efforts to make my annual address as brief as possible it reminds me of a Scotch story. Donald McPherson was a leading member and also a leading deacon in an old church in Scotland, whose old minister had for many years inflicted on his congregation very long and tiresome sermons. One Sunday the old minister was invited to fill the pulpit of a church in an adjoining parish, and Donald’s congregation, thinking this was a good chance to get a much younger man, got one to fill the pulpit for that day. After the services, and as the young minister and Donald were walking home together, the minister naturally asked: “Well, Mr. McPherson, how did you enjoy the sermon?” Donald replied: “Well, minister,” he said, “I think it sounded kind of disconnected, but I liked it awfully well because it was brief.” The young minister was a little frustrated at the frank expression or criticism and replied: “Well, Mr. McPherson, there was perhaps a reason for it being brief and disconnected.” Donald replied: “And what was the reason?” “Well, sir,” the minister stated, “in coming to church this morning, I had occasion to change my manuscript from one pocket to the other, and while doing so, unfortunately, a sudden gust of wind came along and blew several of the pages down the street, and a dog seeing the flying papers got after them, and really, Mr. McPherson, what he didn’t destroy he practically eat up.” Donald, on hearing the excuse, replied: “And so, Mr. Minister, the reason your sermon was brief was because a dog ate it.” “Well,” replied the minister, “yes, Mr. McPherson, that is practically true.” “Well, well,” says Donald, “I will tell you, I am willing to forgive you, and so is all of the congregation, if you will only send a pup of that dog to our old minister.”

The earliest recorded mention of the excuse consisting for a schoolchild in telling that a dog ate their homework is from a speech that, on his retirement from the headmastership, James Bewsher gave on Tuesday 30^th July 1929 to the pupils of Colet Court, London—speech published in The Manchester Guardian (Manchester, Lancashire, England) of Wednesday 31^st July 1929 (Bewsher remarked that the phrase had long been in usage):

“I think that the boys are no worse than they used to be,” said Mr. Bewsher, “in fact I think sometimes they are better. It is a long time since I have had the excuse about the dog tearing up the arithmetic homework. (Laughter.) We have trained the young boys to accept some responsibility and to achieve the power of rising to the occasion when crises happen.”

Frank Fletcher (1870-1954), headmaster from 1911 to 1935 of Charterhouse, a ‘public school’ (i.e. a private fee-paying secondary school) in Godalming, Surrey, mentioned a similar excuse in After Many Days: A Schoolmaster’s Memories (London: Robert Hale and Company, 1937):

He kept a dog, and taught us Greek prose and verse. The two facts are connected in my memory by his occasional apology when he got behindhand with his work, “I’m very sorry, but my dog’s eaten your Greek prose.”

In American English, the phrase must have been already popular in the mid-1950s, since the final exclamation probably alludes punningly to it in the following instalment of Etta Kett, a comic strip by Paul Robinson (1898-1974), published in the Daily Intelligencer Journal (Lancaster, Pennsylvania, USA) of Wednesday 26^th December 1956:

– Mom!! Where’s that fudge pie I whipped up?
– With boys around, that’s a silly question!
– Oh, no!! Not my whole pie!!! – After the way I slaved!
– You dizzy creeps!! I baked that to take to domestic science class tomorrow!!
– You ate my homework!!

A similar punning allusion to the phrase occurs in Restaurant School: What Cooks? Students Do, by William Boldenweck, published in the San Francisco Examiner (San Francisco, California, USA) of Monday 12^th December 1960:

“My little brother ate my homework.”
The excuse has not been tried yet, but it could happen in City College of San Francisco’s hotel and restaurant course, a unique series of classes in which students cook and serve 5,000 meals each school day, punch a time clock and in which part of the “final” is a semi-annual banquet.

A yet similar punning allusion occurs in the following instalment of Blondie, by Murat Bernard ‘Chic’ Young (1901-1973), published in several North-American newspapers on Friday 26^th August 1966—for example in The Leader-Post (Regina, Saskatchewan, Canada):

– What happened to the cupcakes I made for my cooking class?
– I ate those cupcakes
– Boo-hoo, Mommy
– Daddy ate my homework!

Donna Schwab mentioned a variant of the phrase in Underestimation of “Culturally Deprived” Youth, published in New Teachers in Urban Schools: An Inside View (New York: Random House, 1968), by Richard Wisniewski:

Any teacher gullible enough to fall for the inevitable story, “my little sister ate my homework,” without demanding a new version of the same, deserves the reputation she will soon have to live with.