The humorous definition of a banker is of a man who lends you an umbrella when the sun is shining, but who wants the umbrella back the moment it starts raining. That is, a bank will lend you money when you don’t need it and can easily repay the loan, but won’t lend you the money when you do need it. The saying has been attributed to Ralph Waldo Emerson, Mark Twain, Robert Frost and Philip La Follette.
 
“A banker, it has been said jestingly, is a man who lends you an umbrella when the weather is good and takes it back when it rains” has been cited in print from 1930 and is of unknown authorship.
     
 
CafePress
“A banker is a fellow who lends you his umbrella” Gifts
         
Google Books
The International Digest
Volume 1
New York
1930
Pg. 41:
A banker, it has been said jestingly, is a man who lends you an umbrella when the weather is good and takes it back when it rains.
       
Google Books
Bulletin of the Robert Morris Associates
Volumes 15-16
1932
Pg. 84:
New definition of a banker: A person who lends you an umbrella when the weather is fine and asks for it back when it rains.—London Stock Exchange Gazette.
 
Google News Archive
25 February 1932, Lodi (CA) Sentinel, “Did you ever stop to think?” by Edson R. Waite, pg. 2, col. 5:
A lot of money lenders are keeping money from working. They are like the fellow who lends you an umbrella on a sunny day, but asks for it back the moment it starts raining.
 
Google Books
Modern Money:
A treatise on the reform of the theory and practice of political economy
By Lord Henry Ludwig Mond Melchett
London: M. Secker
1933
Pg. 37:
...but unfairly put, that a bank is like a man who lends you an umbrella when it is fine and wants it back when it rains.
 
Google Books
24 October 1934, Ottawa (Ontario) Citizen, pg. 19, col. 1:
A bank is a place where they lend you an umbrella in fair weather and ask for it back the moment it begins to rain.
     
Google Books
Consume! The monetary radical’s defense of capitalism
By Dallas Devello Johnson
New York, NY:  Dynamic America Press
1940
Pg. 86:
It has been appropriately said that “the banks will lend you an umbrella when it is fine weather but always ask for it back again when it rains.”
     
Google Books
The City:
The London financial markets
By Barnard Ellinger
Westminster: P. S. King & Staples
1940
Pg. 223:
Somebody once defined a banker as a man who lends an umbrella when the weather is fine, and asks for its return immediately it begins to rain, and he added that the reason for this is that the umbrella did not belong to the lender.
 
Google Books
Laughing Stock;
Over Six-Hundred Jokes and Anecdotes of Uncertain Vintage
By Bennett Cerf
New York, NY: Grosset and Dunlap
1945
Pg. 86:
Banker: A man who lends you an umbrella when the sun is shining and wants it back the minute it starts to rain.
 
Google Books
If you’re a bank director
By Clarence Godfrey McDavitt
Cambridge, MA: Bankers Pub. Co.
1950
Pg. 37:
LOAN POLICY
Robert Frost, New England’s poet laureat, is quoted as having described a bank as “a place where they lend you an umbrella in fair weather and ask for it back again when it begins to rain.”
           
Google Books
A Second Treasury of the Familiar
By Ralph Louis Woods
New York, NY: Macmillan
1950  
Pg. 222:
Here’s to the banker, who lends you an umbrella when the sun is shining and demands it back as soon as it starts to rain.
   
Google Books
The finance of farming in Great Britain
By Shadrach G. Hooper
London: Europa Publications
1955
Pg. 103:
Those days did much to get rid of their somewhat bitter old definition: ‘A banker is a man who lends you an umbrella when it is fine and asks for it back when it rains.’
 
Google Books
July 1984. American Bar Association Journal, pg. 122, col. 2:
Emerson once said that a bank is an organization that lends you an umbrella when the sky is clear and then asks for it back when it rains.
     
Google Books
America’s Soapbox:
Seventy-five years of free speaking at Cleveland’s City Club forum
By Mark Gottlieb and Diana Tittle
Cleveland, OH: Citizens Press
1987
Pg. 110:
A banker is a fellow who lends you an umbrella when the sun is shining and asks for it back when it rains.
—Philip La Follette, former governor of Wisconsin and son of the late Robert La Follette
 
Google Books
Management Advice from the World’s Great Thinkers
By Jim Miller
New York. NY: Writers Club Press
2003
Pg. 27:
A banker is someone who lends you an umbrella when it is sunny and takes it back at the first sign of rain.
Robert Frost
 
Google Books
New frontiers in banking service :
Emerging needs and tailored products for untapped markets ; with 42 tables
By Luisa Anderloni
Berlin: Springer
2007
Pg. 241:
We are much closer to the maxim attributed to Mark Twain: “A banker is someone who lends you an umbrella when the weather is fine, and takes it away when it starts to rain”.
 
Moneyweb
13 October 2010 23:01
Special report podcast: Richard Pike - CEO, Adcorp
(...)
ALEC HOGG: It also emphasises that old saying that the banks will give you an umbrella when the sun is shining and then want to take it away when it’s raining.
RICHARD PIKE: Tell me about it [laughing], they’ll only lend you money if you can prove that you don’t need it.