Of British-English origin, the phrase that, or the, Kruschen feeling denotes great vitality, enthusiasm and liveliness.

This phrase was originally used in the advertisements for Kruschen Salts.

It was the Kruschen feeling that was used in the earliest advertisement for Kruschen Salts that I have found, from the Yorkshire Evening Post (Leeds, Yorkshire, England) of Thursday 10^th March 1921:

Cheerio!
—the Kruschen feeling—ready for anything! Kruschen keeps you fit. Just half a teaspoonful of Kruschen Salts in a tumbler of hot water every morning——enough for two months in a bottle 1/9 of all Chemists
Kruschen Salts

It was that Kruschen feeling that was used in the second-earliest advertisement for Kruschen Salts that I have found, published in the Daily Herald (London, England) of Wednesday 15^th March 1922:

That “Kruschen” Feeling

Here’s the dear old boy again—just brimming over with high spirits and the sheer joy of living.
He gets up every morning feeling that he could do the staircase in one. He greets the day with a smile, and his friends with the hearty hand-clasp of real good-fellowship.
He is welcome wherever he goes—no one can resist his cheery way of always looking on the bright side of things.
Do you feel like this?
If not, there’s something wrong. You think it’s your nature. It isn’t—it’s your health.
Just consider. You are probably getting too little fresh air and exercise—perhaps you spend too much time sitting over your daily tasks, and, as a result, you get run down with worry, overwork or errors of diet. Consequently, you need a corrective. You need Kruschen Salts.
Kruschen contains just the six salts, blended in just the right proportion, that are essential to keep you in continual good health. Under their tonic influence the liver and kidneys eliminate all waste matter. Your whole body is braced and revived.
The dose is only “as much as will lie on a sixpence,” and you can’t taste it in your breakfast cup of tea. A 1/9 bottle contains 96 morning “pinches”—good health and youthful spirits for less than a farthing a day!

The earliest occurrence that I have found of that, or the, Kruschen feeling used as a phrase is from Letters from Leicestershire and elsewhere, published in The Tatler (London, England) of Wednesday 6^th December 1922:

On Tuesday the Cottesmore were at Whadborough Cross Roads. But having exchanged a comfortable seat on my horse for a very hard one on the floor, and had to return for repairs at the base, I know very little about what happened. Beware of horses with that Kruschen feeling. Hounds had a very nice day with a good hunt from Orton Park in the evening.

The second-earliest occurrence that I have found of that, or the, Kruschen feeling used as a phrase is from The Wells Journal (Wells, Somerset, England) of Friday 8^th December 1922:

WELLS DECEMBER FAIR.
DECLINE IN VISITORS AND AMUSEMENTS

Whether due to trade depression or to fading interest in old-world events, Wells December Fair on Tuesday lacked much of its former attractiveness, the crowd of visitors being decidedly smaller, and the extent of the amusement much diminished. […]
[…]
Upon the rows of homely cocoanuts, awaiting disturbance from their lonely pillars by viciously thrown balls sad eyes were cast, and heavy-hearted rural representatives passed them by with a reminiscent sigh. Brightly-hued celluloid balls fretfully fidgeted on tiny jets, and occasionally a kindly souled farmer would take compassion on their jerky appeal, and by the aid of a miniature rifle would lay them at rest in their wire basket. Five sepulchral pins did their best to dispel the prevailing gloom, and periodically some burly fellow, animated doubtless with the “Kruschen feeling,” would upset the equilibrium of four, and with the stoop of “the man with the hoe” would reluctantly take away, as his reward, a packet of cigarettes.

The phrase then occurs in The Week’s Football, published in The Hastings and St Leonards Observer (Hastings, Sussex, England) of Saturday 20^th January 1923:

“Let’s have some cheerful stuff,” said one of the Hastings supporters to us this week. Well, we must say, we have “that Kruschen feeling.” The French may enter Essen, Bochum or Timbuctoo, but the news which shook the firmament last Saturday was that the Signals’ home record had been bent, broken and blasted.

In Contemporary English: A Personal Speech Record (Leipzig and Berlin: B. G. Teubner, 1927), the British linguist William Edward Collinson (1889-1969) mentioned the Kruschen feeling in a paragraph about the influence of advertisements on literature:

At times advertisements exert their influence even on the language of literature as where Galsworthy, (p. 129) ¹ speaks of a certain shade of colour of the eyes as Reckitt’s blue (cf. H. H. Richardson, The Way Home p. 113 ²: “as for the sky, Mahony declared it made him think of a Reckitt’s bluebag”) from the blue-bags commonly used in washing clothes and manufactured by the firm of that name. When Wodehouse mentions that schoolgirl complexion (Ukridge 240) ³, many of us will at once think of the advertisement of Palmolive soap. It is almost as distinctive as the Kruschen feeling (being full of zip and go) from Kruschen salts.

¹ The English novelist and playwright John Galsworthy (1867-1933) used Reckitt’s blue eyes in The White Monkey (London: William Heinemann Ltd., 1924), the first novel in the second Forsyte trilogy, entitled A Modern Comedy.

² Henry Handel Richardson was the pen name of the Australian novelist and short-story writer Ethel Florence Lindesay Richardson (1870-1946).

³ The English novelist and short-story writer Pelham Grenville Wodehouse (1881-1975) used the phrase that schoolgirl complexion in Ukridge Rounds a Nasty Corner, published in the collection of short stories Ukridge (London: Herbert Jenkins Limited, 1924).