A shortened form of grotesque, the informal British-English adjective grotty is a general term of disapproval, meaning unpleasant, dirty, nasty, ugly, etc.

The adjective grotty is first recorded in—and was popularised by—A Hard Day’s Night (1964), a musical comedy film directed by Richard Lester (born 1932), starring the Beatles ¹—grotty occurs in this dialogue between George Harrison, who has been mistaken for a teen model, and the trend-setter Simon Marshall, interpreted by the British actor Kenneth Haigh (1931-2018):

Marshall : We’d like you to give us your opinion on some clothes for teenagers.
George : Oh, by all means. I’d be quite prepared for that eventuality.
[…]
Marshall : Well, at least he’s polite. Show him the shirts, Adrian.
[Adrian hands two shirts to George.]
Marshall : Now, you’ll like these. You’ll really “dig” them. They’re “fab,” and all the other pimply hyperboles.
George : I wouldn’t be seen dead in them. They’re dead grotty.
Marshall : Grotty?
George : Yeah, grotesque.
Marshall : Make a note of that word and give it to Susan. It’s rather touching, really. Here’s this kid, trying to give me his utterly valueless opinion, when I know for a fact that within a month, he’ll be suffering from a violent inferiority complex and loss of status, because he isn’t wearing one of these nasty things! Of course they’re grotty, you wretched nit! That’s why they were designed! But that’s what you’ll want.
George : I won’t!

¹ The Beatles were a pop and rock group from Liverpool, consisting of George Harrison (1943-2001), John Lennon (1940-1980), Paul McCartney (born 1942) and Ringo Starr (Richard Starkey – born 1940).

Alexander Walker mentioned grotty in the review of A Hard Day’s Night, published in the Evening Standard (London, England) of Monday 6^th July 1964:

The WITTIEST bit is George Harrison being polled for his opinions on teenage fashions by a frantic trend-spotter, well played by Kenneth Haigh, who is shaken to hear him denounce his new shirt designs as “grotty”—Beatle-talk for “grotesque.”
“Could he be an early clue to a new direction?” ponders Haigh—then rushes to a wall chart and gasps with relief, “No he’s not due for three weeks, yet.”

According to the newspaper accounts of the London premiere of A Hard Day’s Night, published on Tuesday 7^th July 1964, Antony Armstrong-Jones ² asked George Harrison to explain the meaning of grotty—the following, for example, is from the Daily Mirror (London, England):

“I don’t think we’re very good, Ma’am,” says Paul.
A ROYAL ‘SHAKE’ FOR RINGO

Princess Margaret ³ shakes hands with Ringo Starr as she meets the Beatles in the foyer of a London theatre last night.
She arrived with her husband Tony Armstrong-Jones, and was met by 12,000 screaming, chanting fans who jammed Piccadilly-circus.
The occasion? The London Pavilion premiere of “A Hard Day’s Night”—the four boys’ first film.
First, the Princess shook hands with drummer Ringo. John Lennon, Paul McCartney and George Harrison looked on.
Then she asked Paul: “What do you think of the film?”
Said Paul: “I don’t think we are very good, Ma’am. But we had a very good producer.”
Next Tony asked George: “What is the meaning of that phrase you use in the film—‘That shirt is very GROTTY’?”
And George explained that “grotty” is the with-it word for GROTESQUE. Tony laughed—and said he had seen the word in newspaper reviews.
As soon as the Princess and her husband had left the foyer the girl programme sellers—wearing short evening dresses featuring Beatle pictures—asked the group to sign their DRESSES.

² Antony Armstrong-Jones (1930-2017) was a British photographer and filmmaker; he married Princess Margaret in 1960; they divorced in 1978.
³ Princess Margaret (1930-2002) was the only sister of Elizabeth II.

The adjective grotty then occurs:

1-: In London fashion collections, by Margaret Williams, published in the Cheshire Observer (Chester, Cheshire, England) of Friday 24^th July 1964:

You will love the coppery coloured stockings; the white-legged look has gone, thank goodness, and the few girls I saw sporting it in London this week looked rather grotesque (or “grotty” as the Beatles would say).

2-: In The headache that faces Lord Harewood, by Sydney Edwards, published in the Evening Standard (London, England) of Tuesday 18^th August 1964:

Edinburgh, Tuesday.
They’re all “dead grotty,” in Edinburgh today, particularly Joan Littlewood and her Theatre Workshop Company.
No one was hiding the fact that they came a cropper in their Festival production of Henry IV except George Cooper, who said like a good Falstaff, “I thought things went very well.”
But most of the festival’s 90,000 visitors have that grotty feeling for one or another reason: The rain has been coming down non-stop ever since the festival opened three days ago.

3-: In Man’s world: No grotty gear, by David Morton, published in the Tatler and Bystander (London, England) of Wednesday 26^th August 1964:

Turning off Regent Street by Jaeger’s (and pausing, I suggest, to see the camel-coloured blazers and double-breasted motoring coats) a man finds himself in Fouberts Place, where every second shop seems to sell sandwiches. Taking the second turning right, he’ll be in Carnaby Street—Modsville, W.1. […]
[…]
This is the street where you can walk in looking “grotty” (Beatle-ese for, and derived from, grotesque) and walk out looking “gear.”