The British- and Irish-English phrase to spend money as if it were going out of fashion means to spend money recklessly, as if it were either worthless or soon to become so.
—Irish-English synonym: to throw money around like snuff at a wake.

It was perhaps originally a Liverpudlian phrase, since all the earliest uses that I have found occur—as a misogynistic cliché repeated over and over again—from 1962 to 1967 in the Liverpool Echo (Liverpool, Lancashire, England):

1: On Monday 24^th September 1962, in Echoes and Gossip of the Day:

Pounding Away
Some women spend money as if it were going out of fashion.

2: On Saturday 14^th March 1964, in Echoes Of The Day:

Club member.—My wife spends money as if it were going out of fashion.
Spinster.—The trouble is that men don’t start overlooking your age until you’re looking your age.

3: On Monday 22^nd June 1964, in Echoes and Gossip of the Day:

Club member.—My wife spends money as if it were going out of fashion.

4: On Monday 19^th July 1965, in Echoes and Gossip of the Day:

Pounding Away
Club member.—My wife spends money as if it were going out of fashion.

5: On Saturday 29^th October 1966, in Echoes and Gossip of the Day:

Fashion Notes
“My wife spends money as if it were going out of fashion.”

6: On Friday 22^nd September 1967, in Echoes and Gossip:

Overheard: My wife spends money as if it were going out of fashion.

The earliest use of the phrase in actual speech that I have found is from ‘Keep it dark,’ said the man. ‘We’re on to a good thing here.’ William Marshall introduces the Mirror’s 4-page salute to Swansea, published in the Daily Mirror (London, England) of Monday 21^st August 1967:

The Swansea area is humming and pounding with industry and the good, if not the sweet, things of life.
There are 16,000 workers at the Steel Company of Wales’s Port Talbot plant alone, all averaging more than twenty quid a time, which tots up to £320,000 a week banging and clanging on the cash registers.
An English businessman in Swansea for twenty-five years says: “In the old days here, they would struggle all their lives to send their sons to university.
“Nowadays, with most education free, they spend money like it’s going out of fashion. They want two cars and holidays in Italy—and a good drink.”

The phrase also occurs in Open Champion wants to make a million, but—Is Rocket Lee burning fuel too fast?, a portrait by Jack Magowan of the U.S. golfer Lee Buck Trevino (born 1939) published in the Belfast Telegraph (Belfast, County Antrim, Northern Ireland) of Monday 17^th July 1972:

To-day, Lee is like a kid in the sweet shop. He never sits still, chats endlessly to everybody, and spends money like it was going out of fashion. “Why not. I’ve already more in the bank than I can count,” he boasts happily.

The earliest occurrence that I have found of the variant to throw money around as if it were going out of fashion is from a letter published in the Evening Express (Aberdeen, Aberdeenshire, Scotland) of Saturday 30^th July 1988—Context: A record £2-million transfer fee had been paid in the summer of 1988, when Tottenham Hotspur signed Paul Gascoigne from Newcastle United; within weeks of Gascoigne’s transfer, the national record was broken again when Tony Cottee moved from West Ham United to Everton for £2.5 million. As manager of the Rangers, a Glasgow football team, the Scottish football player Graeme Souness (born 1953) had signed a lot of players from English clubs:

Souness must take part of blame
The whole world’s going crazy reckons Ian Hendry, Westholme Avenue, Aberdeen. He writes:
The record-breaking £2.5 million transfer of Tony Cottee following hard on the heels of Gascoigne’s £2 million move to Spurs is evidence the market is again going crazy.
Graeme Souness must shoulder some of the responsibility as he continues to throw money around as if it were going out of fashion.
Souness must be the only person in Britain who thinks a full back, especially one as overrated as Gary Stevens, is worth a million.
With this type of loadsamoney approach prices will continue to spiral to the detriment of football as a whole.
The top clubs will continue to cream off the best talent.
It’s a bit sweet and Souness.