"When in doubt, make it stout” is a mechanical engineering adage that has been printed on many images. It’s not known where the saying originated.

“When in doubt, make it stout” was printed in the book Man and the Technological Society (1973) by L. Daniel Metz and Richard E. Klein. “‘When in doubt make it stout out of the stuff you know about” was printed in the book Selection of Materials and Manufacturing Processes for Engineering Design (1989) by Mahmoud M. Farag. “Remember the civil engineer who said, ‘When in doubt, make it stout with materials you know about’?” was posted on Twitter by Jason Call on August 19, 2015.


Google Books
Man and the Technological Society
By L. Daniel Metz and Richard E. Klein
Englewood Cliffs, NJ: Prentice-Hall
1973
Pg. 152:
However, recall that tall skyscrapers do involve vast sums of money, and that dynamic oscillations are the limiting factor on many tall skyscrapers (the price of some buildings could be doubled if one uses the “when in doubt, make it stout” philosophy), ...

Google Books
Decision Analysis and Damage Evaluation in Seismic Design
By Thomas Andrew Sabol
M.S. thesis
UCLA School of Engineering and Applied Science
1982
Pg. 1:
A structural engineer, confronted by a structural design problem with which there is some uncertainty, is apt to adopt the rule: “when in doubt, make it stout.” Evidently the rationale is that a little additional money contributed by the client during the construction phase is preferable, in the engineer’s opinion, to an increased likelihood of unsatisfactory structural performance once the structure is completed.

Google Books
Selection of Materials and Manufacturing Processes for Engineering Design
By Mahmoud M. Farag
New York, NY: Prentice Hall International, Incorporated
1989
Pg. 417:
It is often said ‘when in doubt make it stout out of the stuff you know about’.

25 May 1989, Star Tribune (Minneapolis, MN), “Technology: Simulation testing beats the real thing” by Steve Gross, pg. 1D, col. 3: 
Before computers were used in designing farm tractors and construction equipment in the late 1970s, there used to be a rhyme about how to build the equipment.

It went like this:

“When in doubt
Make it stout
Out of the thing
You know about.”

Google Groups: rec.woodworking
Figuring center of gravity?
Dave Campbell
5/2/96
(...)
Please no advice like “when in doubt, make it stout"--it need to be proportional and not top-heavy.

Google Groups: sci.engr.manufacturing
Help Wanted: Machine for Placing Insulation on Copper Pipe
Peter Berghs
3/2/97
(...)
When in doubt
Make it stout
Out of something
You know about

Twitter
Jake
@ikantthink
Structural engineer: “When in doubt, make it stout. Stiffness attracts load.”
1:12 PM - 29 Jan 2009

Twitter
Grant Johnson
@thegrantone61
“When in doubt, make it stout.” - Kyle Froetschel #homework11 #FOS #FeedbackSystems
12:52 AM - 29 Nov 2012

Twitter
Jason Call
@JasonCall4
Remember the civil engineer who said, “When in doubt, make it stout with materials you know about”? Thank him for the next sinkhole you see.
1:32 AM - 19 Aug 2015

Twitter
Antonio Fedele
@iamtheregulator
When in doubt
Make it stout
(stress engineer rule #1)
#Kodak #Trix400 #Mamiya #Mamiya645 #… http://ift.tt/2nmODmW
6:39 AM - 27 Mar 2017

Twitter
M.C. Oen 🇺🇸
@M_Calf_Oen
When in doubt make it stout… applicable no only in beer decisions but engineering and life decisions. #StoutLife
3:28 PM - 30 Apr 2019