In the colloquial British-English phrase bob-a-job:
– the noun bob denotes a shilling, i.e., a former monetary unit of the United Kingdom, equal to one twentieth of a pound or twelve pence;
– the noun job denotes an isolated or casual piece of work, undertaken for a one-off payment or on a hire basis.

The phrase bob-a-job occurs, for example, in the following from the column City Spy, published in the Evening Standard (London, England) of Monday 19^th July 2021:

Beelivery looks for bob-a-job bikers with app and own cash
RAPID grocery delivery start-up Beelivery is spending £2 million on TV ads as it recruits 50,000 new couriers to join the team. It works thus: an order lands on the worker’s smartphone, who buys a customer’s shopping (from a shop) with their own cash, drops it off using their own bike and waits for the expenses plus up to £10 per delivery to land in their account. In essence, bob-a-job with an app. It’s only a matter of time before Beelivery, above [cf. note], becomes the next “unicorn” with a multimillion dollar valuation from a Series D funding round before being taken public via a Spac for a squillion dollars. Paging Softbank’s Masa Son…

Note: “above” seems to refer to the following from the same column:

PERHAPS Beelivery’s customers will have more luck than those of Ocado. Thousands of orders have been cancelled or delayed after a three-droid pile-up at its robot packing warehouse in Erith. For fans of JG Ballard’s future dystopias, this is just the beginning.

Originally, the phrase bob-a-job was the slogan of the Boy Scout Association’s effort to raise money for funds by doing jobs, at a shilling a time.

The first two occurrences of this phrase that I have found are from newspapers published on Friday 19^th May 1944:

1-: From the Midland Counties Tribune and Warwickshire County Graphic (Nuneaton, Warwickshire, England):

Boy Scouts’ Associations’ Novel Scheme
Nuneaton Troops’ Target in “Bob-a-Job” Workday
(“Tribune” Special.)

Boy Scouts’ Association all over the country—including the Nuneaton and District Association—will take part during the coming week-end in an ingenious scheme to raise money for the Boy Scouts’ Relief Abroad Fund. The scheme which will be held on Saturday, has the engaging title of “the bob-a-job workday.”
Variety of Jobs.
Rover Assistant District Commissioner John Croydon, of 63, Bracebridge Street, Nuneaton, told a “Tribune” representative that letters had been sent out to members of the public stating: “The Boy Scouts’ Association is sending volunteer teams of Scout-trained men and women to serve with the relief forces abroad. This volunteer venture of succour to the starved and stricken victims of the war-devastated countries will cost a lot of money. As the funds of the Boy Scouts Association cannot be used for this purpose the Wolf Cubs and Scouts have undertaken to raise the required sum by a nation-wide day of work on Saturday, May 20, 1944. Every Wolf Cub and Scout will do a job of work on that day and give the money earned to the fund.
“Will you give a Cub or Scout a job on that day? Anything a boy can do such as weeding a garden, mowing lawns, cleaning windows, cleaning silver or other odd jobs about the house, fetching shopping, delivering parcels, chopping wood or sawing logs, etc. You pay whatever you think his services are worth, remembering, perhaps, the purpose for which he is working. He is working because it is a principle of the movement that on the part of the boys themselves money must be earned and not solicited.”
Chief Scout’s Call.
The Chief Scout (Lord Somers), said Mr. Croydon, had issued a statement in which he had said: “May 20, 1944, will be a day that every Scout in every troop is going to remember, just as every man who was a Scout in 1914 remembers Saturday, May 2^nd, in that memorable year. On May 2, 1914, 200,000 Scouts did a job of work and gave the money earned to the National Institute for the Blind. On May 20, 1944, I want every Cub and Scout to earn at least 1s. If the 424,000 of us in the British Isles contributes an earned shilling we shall raise the magnificent sum of £21,200 by this one day effort.”
Nuneaton’s Target £30.
Mr. Croydon added that the Nuneaton Scouts’ target for Saturday was £30, and the Association hoped that the public would co-operate by offering jobs to be done. The object was a worthy one, and Scouts and Cubs had every confidence that their services at “a bob-a-job” would be fully utilised. All the money raised during the day would be handed over to a prominent Nuneaton citizen at a gathering of Scouts and Cubs between 7 p.m. and 8 p.m. the same evening.

2-: From the Leicester Mercury (Leicester, Leicestershire, England):

Scouts Want Paying for ‘Good Turns’ To-morrow

Scouts all over the country to-morrow will depart from one of their oldest rules, and instead of doing their daily “good turn” for nothing, will ask for at least a shilling for the funds of their Abroad Service.
“A date with a job,” and “A bob a job” will be their mottoes, and in the four areas of Leicester, East, West, North and South, they will be competing against each other in raising the money.
Every Scout and Cub is asked to make a special effort to-morrow, and the public are requested to assist the lads by seeing that jobs are available. Suggested tasks for the boys are gardening, minding children, shopping, and housework.
The Scouts Abroad Service is ready to send parties to the Continent as soon as possible to help in the war-stricken areas, but money is needed.