The colloquial British-English phrase to laugh like a drain means to laugh loudly and coarsely.
This phrase alludes to the sound of water gurgling down a drain.
These are the earliest occurrences of the phrase to laugh like a drain that I have found, in chronological order:
1 & 2-: From the column Petrol Vapour, by John Oliver, published in The Tatler (London, England):
1-: Of Wednesday 11th December 1935—John Oliver was reviewing You Have Been Warned: A Complete Guide to the Road (London: Methuen & Co. Ltd., 1935), by the Irish-born British author and broadcaster William Donald Hamilton McCullough (1901-1978) and the British cartoonist Fougasse (Cyril Kenneth Bird – 1887-1965):
The book does not make you roar with mirth like a foghorn, roll about the carpet kicking over the chairs and tables and gnawing at the domes of silence. Nor does it make you laugh like a drain; nor yet like a cistern or a sewer or any of the other things people seem to laugh like nowadays—not even like T. S. Eliot’s Mr. Apollinax *, who, you’ll recall, in the palace of Mrs. Phlaccus, at Professor Channing-Cheetah’s, laughed “like an irresponsible fœtus.” No. It does not make you laugh as I imagine an irresponsible fœtus would laugh, in a roystering and marine manner. Rather it produces sharp, pointed, piercing laughter. One laughs, I’d say, in the manner of an exceedingly small steam safety-valve with a rather restricted aperture. And although such non-Miltonic laughter, owing to the enormous pressures produced around the orifice, is excruciatingly painful, I recommend it for its health and sanity-giving properties.
* Mr. Apollinax is one of the twelve poems included in Prufrock and Other Observations (London: The Egoist Ltd, 1917), by the U.S.-born British poet, critic and playwright Thomas Stearns Eliot (1888-1965).
2-: Of Wednesday 15th July 1936:
Petrol is the picnic spirit. It enables the preliminaries to an open-air eating to be got over expeditiously. It enables the Thermos and the sandwiches to be transported to fresh woods and pastures new with a minimum loss of time; and, later, it enables the wasps to be outdistanced in the race for home. It is to petrol that we must attribute the revival of alfresco ingestion in its most entertaining form. But first, what is a picnic? I’d define it as an alimentary obstacle race. The paraphernalia of food are there; but between the cup and the lip, the tongue and the tinned tongue, many difficulties are interposed. Some of them are due to the adoption of the semi-supine stance. One eats and drinks at one’s peril, and it’s rare that a glass is drained without some mirth-provoking confusion between œsophagus and trachea—mirth-provoking, that is, for the spectators. (My dear! he looked just as if he were going to burst a blood-vessel; haw, haw; I laughed like a drain.)
3-: From Another “Flash” Bentley Story: “Some Chick—Some Peck!”, by Peter Wilson, published in the Sunday Pictorial (London, England) of Sunday 31st May 1942:
Sergeant Legg was just finishing with Lefty McKane. “I’ll teach you to know your left from your right, you great Canadian slum-mock. There’s about a ton of potatoes to be peeled and by the time you’ve finished peeling them you’ll think of spuds every time I give you ‘eyes right’!”
And, with that, Legg dismissed the squad and bounced his way across the square to the Sergeants’ Mess. He was bullying one of the orderlies when Flash came in and he broke off, rather pointedly, to call across the room.
“Today’s the last day for entries for the boxing, isn’t it, Bill. I don’t suppose I’ll get a scrap. The boys all know me—and they know I don’t play light!”
Flash moved over to the notice-board. Then he drawled:
“No, you don’t get a walk-over, Legg. The list’s just come out from Orderly Room and there’s a pal of yours in the welters—Private McKane.”
Legg laughed like a drain.
“That lunkhead,” he guffawed. “I’d eat him before breakfast—without my sugar ration.”
4-: From Immortals of the Squadrons—2, by John Cashel, published in John Bull (London, England) of Saturday 5th December 1942:
Every unit of the R.A.F. has its own cherished memories. This series presents a gallery of born fighters whose exploits will never be forgotten in any club or mess where flying men foregather . . .
Early one morning at a fighter station near London, a man passing a bathroom cubicle saw “Paddy” Finucane rubbing himself down and gave him a friendly flick on the back with a towel.
Paddy popped his head out of the doorway in time to see somebody going into another bathroom.
A fellow pilot, he thought.
So he filled a jug with cold water, slipped along the corridor, threw the cold douche over the top of the other cubicle and fled as a voice yelled out in protest.
It was the voice of the Group Captain.
Paddy, apologising at breakfast, was told, “Don’t be silly. I laughed like a drain.”
5-: From Side Lights, published in The Journal (Ottawa, Ontario) of Monday 21st February 1944:
WO. Keith Campbell Over the BBC.
—“One time I was coming back after photographing the bomb damage of Dusseldorf when two Focke-Wulfs came in to attack me. I went into a circle and kept turning as sharply as I could. The Focke-Wulfs did the same, but because I was turning in a smaller circle than they were able to do I eventually landed on their tails. They were flying side by side, and the right-hand bloke must have caught sight of me in his mirror and he obviously thought I was armed because he peeled off into a deep dive and disappeared. When his mate saw him go he must have thought I’d shot him down, for he pushed off like a bat out of hell. I laughed like a drain, sitting there in my cockpit with only a Very-pistol, and having sent two gallant Luftwaffe pilots running for their lives.”
6-: From the review of Vespers in Vienna (Boston, Massachusetts: Houghton Mifflin Company, 1947), a novel by the Scottish author Bruce Marshall (1899-1987)—review by Elizabeth Clarkson Zwart, published in The Des Moines Register (Des Moines, Iowa) of Sunday 24th August 1947:
The British slang—or is it?—which seeps into even the nuns’ mouths, well might plant new words in the vocabularies of young American readers:
Entertainingly, everything good is “wizard;” anyone who is not particularly bright “hasn’t a clue” or he is a “dim bulb;” hungry and thirsty Britishers take “lashings” of salt in their soup and “lashings” of brandy in their soda; people don’t know other people “from a bar of soap;” intrigued men go “swanning around” the seductive girls; and people have a laugh “like a drain” or everything is “going down the drain” or should be poured “down the drain”.
7-: From A Dictionary of Forces’ Slang, 1939-1945 (London: Secker & Warburg, 1948), by Eric Partridge, Wilfred Granville and Frank Roberts:
Laugh like a drain, to chuckle ‘consumedly’; laugh loudly, especially at someone’s discomfiture. (Ward-room and also Army officers’.)
8-: From the caption to the following cartoon, published in The Tatler and Bystander (London, England) of Wednesday 18th May 1949:
My Aunt — by Mallet
My Aunt had a Chaffer [sic] named Nash
Who drove with abandon and dash . . . . .
He would weave the old Rolls
Through the cart-ruts and holes –
And laugh like a drain at the splash!