What does it mean to "retire a risk"?
The earliest examples I have found so far are:
Mr. GOLDIN [NASA Administrator] ... I want to solve this specific problem. I want to make a casting with small grain size and high strength. I want to understand the mechanical characteristics of the ball bearings. I want to take the vibrational problems out of the system. You could do that. You could retire the risk, if you will, and then proceed on with the program and not have the type of problems that we have had. US Congress Hearing, NASA Fiscal Year 1993 Oversight
(Note the if you will in the above, perhaps suggesting the speaker thinks the expression may be unfamiliar to the audience.)
... get the money to build a follow-on to the Shuttle. We can make it possible for the industry to do that. We are trying to do that in two ways. The first is to reduce the technical risk with technology programs that will allow industry to conclude with a high degree of certainty that the risk has been retired. Aerospace America, vol. 33, p.12 (1995, Google Books snippet)
This terminology in Risk Management, which appears to have begun in the aerospace industry, is used by the US Department of Defense:
Communicating and Feedback Process – Process for communicating the status of potential, current, and retired risks as well as opportunities that may exist to all personnel involved in risk management (p.9)
...
Once a program team has determined that the mitigation strategy for a risk is to control it, part of the control plan may include a risk burn-down plan for high risks. For most risks the burn-down plan consists of steps, tied to the project schedule, that allow the program to control the risk and retire risks. (p.36)
DOD Risk Management Guide for Defense Acquisition Programs (2014)
I was led to the DOD Guide from this webpage:
Question
Is there any template of questions or criteria that could be used for a risk to meet in order to be realized, retired, or identified?
Answer
This is an excellent question because it gets at the heart of good risk management. In order to answer it, we should first discuss the Program Risk Process (PRP) in general. ...
Risk Identification. Per the DoD Risk, Issue, and Opportunity Management Guide (RIO Guide), Jan 2017, risk identification occurs after the process planning step.
Risk Realization. Risk realization means the risk is no longer a risk (something that may occur in the future) since it has come to fruition. Just ask, did this risk actually occur? If so, it is an issue (assuming it is problematic). ...
Risk Retirement. There are multiple ways to retire a risk or an issue. It becomes a judgment call as to the appropriate time to take it off your RIO rhythm. You could consider questions like: Does this risk warrant any more tracking or mitigation actions? Has this risk come to fruition and is no longer an issue? DAU company webpage
The use of the term risk by NASA is specific and may be peculiar to NASA.
See https://www.nasa.gov/seh/6-4-technical-risk-management.
Because NASA’s own documents define their use of the term, their own definition of risks (and the retirement thereof) is paramount in any use of the term by them.