The Australian-English phrase not worth a crumpet means utterly worthless.

It is one of the phrases built on the pattern not worth a —, such as not worth a tinker’s curse.

The earliest occurrences of the phrase not worth a crumpet that I have found are as follows, in chronological order:

1-: From We Were the Rats (Sydney: Angus and Robertson Limited, 1944), about the siege of the Libyan port of Tobruk in 1941, by the Australian author and journalist Jack Lawson Glassop (1913-1966):

I liked Doug Jackson. He was a Great Public Schools boy and even I had suspected him, his English accent and his rapid promotion, at first. “He’s a bloody cissy,” said Eddie. “You mark me words, he won’t be worth a crumpet in action, not worth a bloody crumpet.” And nearly all the others agreed with him.

2-: From the account of a rugby match between St. George Union and Randwick Union, published in Truth (Sydney, New South Wales) of Sunday 31^st August 1947:

The Saints’ centres, Campbell and Hubble, were not worth a crumpet as a defensive unit against Crowe and Cox.

3-: From the account of a match between Boys’ Club and Myall Creek, in the Inverell District cricket competition, published in The Inverell Times (Inverell, New South Wales) of Monday 16^th March 1953:

Darby and Mathews opened for the Boys and a good stand it was, too. Darby scored 35 and Mathews shared an 81 partnership with him. First drop Witherden added a solid 37—but it was just as well the first three did so well, for the rest were not worth a crumpet—five ducks, a six and one.

4-: From the caption to the following cartoon, published in Pix (Sydney, New South Wales) of Saturday 15^th August 1953:

WITCHETTY’S TRIBE
By JOLIFFE

“All this, as far as the eye can see, is aboriginal reserve by white man government decree—
—which means it isn’t worth a crumpet!”

5-: From The Martinet, a short story by W. E. McGregor, published in The Sun (Sydney, New South Wales) of Friday 11^th September 1953—the following is about female soldiers:

Critchley re-entered the fray.
“But Major,” he said, almost apologetically, “surely they did a good job in the last war.”
“Confound me for a dolt, if the man’s not raving again,” thundered the Major. “Just what you’d expect from a bachelor! Critchley, they weren’t worth a crumpet. Give me one thieving Pathan to fifty of your lady soldiers.”

There have been extended forms of the phrase not worth a crumpet. These are three examples:

1-: From The Pittsburgh Press (Pittsburgh, Pennsylvania, USA) of Wednesday 18^th September 1957:

Titled Woman Visitor Disproves a Theory
Lady Russell, Here to See Zionists, Belies Idea Royalty Wastes Time

The American idea that titled English women do nothing more than sit around sipping tea isn’t worth a crumbled crumpet.
At least not so far as Lady Russell of Liverpool is concerned.

2-: From What teacher wanted to say—less politely, by a teacher named Kerry Cue, published in The Age (Melbourne, Victoria) of Friday 11^th February 1983:

The students who “lack socialisation skills” are a real worry. They pinch other kids’ pens, pick fights with anyone in a ruler-wielding radius and—I’m not sure how to put this—have the unique talent of being able to break wind on command to the inward delight and outward horror of the rest of the class.
Of course “socialisation” isn’t worth a canteen crumpet if your son “lacks a conceptual appreciation of the classroom situation”, which implies he is constantly leaning back on his chair, knocking his mate’s books to the floor; and when sent to the back of the room, he grabs various kids’ rulers on the way, so that he can spend the rest of the lesson karate-chopping two-centimetre bits off the end of each one.

3-: From The Canberra Times (Canberra, Australian Capital Territory) of Friday 3^rd August 1990:

Goss tables guidelines for foreign investment

BRISBANE: The Queensland Government reaffirmed its foreign investment guidelines yesterday […].
[…]
The Deputy Opposition Leader, Rob Borbidge, responded by saying the alleged guidelines tabled by the Premier were not worth a “cold crumpet” and would only generate more confusion.