What does “Buckle up” mean when "Trump and allies go on war footing"?

There was the following sentence led by the headline, “Buckle up: As Mueller probe enters second year, Trump and allies go on war footing.” It follows:

“The investigation has already delivered indictments or guilty pleas involving 10 people and three companies. Many of President Trump’s aides and associates say they are confident the president will be exonerated. But they privately worries that the probe may yet ensnare more figures in Trump’s orbit.”

Cambridge Dictionary simply defines “buckle up” as; u.s. informal. to fasten the belt that keeps you in your sheet in a car or plane. u.k. = belt up.

Oxford Advanced Learners’ English Dictionary defines it as; 1. to fasten sth, or be fastened with a buckle. 2. (AmE) belt up at belt.

Freedictionary defines it as; (slang) prepare for what is about to happen, such as danger, excitement, trouble.

I wonder if it could be interpreted as "brace for a challenge / critical situation."

What does “buckle up” here exactly mean?

Solution 1:

"Buckle up!" is an instruction based on a verb. "Put your seatbelt/safety belt on!" It refers to, and is a metaphor for, the seatbelt buckle in a vehicle.

It is often followed by "it's going to be a bumpy ride." A seatbelt in a vehicle must be worn for our safety. Similarly, we (and/or others) must prepare for difficult times ahead when we read such headlines.

By wearing our metaphorical seatbelt, we are reducing the risk of figurative injury which may be caused by what the headline is predicting or of what detrimental consequences it is warning us about.

It was said to the two children in the movie Back to the Future III, just before the train departed.

Solution 2:

Americans of a certain age will recall that when seat belts became available as a common extra-cost option, but shortly before the government made them mandatory on new cars (around 1964), the U.S. Ad Council heavily promoted a public-service advertising campaign urging automobile drivers and passengers to buy and use seat belts. The name of the campaign was "Buckle Up for Safety," and the main TV advertisement featured a jingle of the same name that ended with the exhortation, "Buckle up!" I hadn't seen this ad in 50 years when I looked it up just now, but I still remembered parts of the jingle word for word. That ad, I think, is the source of "Buckle up!" as a catch-phrase.

Prior to the "Buckle Up for Safety" ad campaign, the more common seat belt expression/warning was simply "Fasten your seat belts"—typically issued as an instruction by stewardesses at the beginning of airline flights, and made famous by Bette Davis's line in All About Eve (1950), "Fasten your seat belts; it's going to be a bumpy night!" (The airline instruction sometimes cautioned about "a bumpy flight"—not "a bumpy night."

In answer to Yoichi Oishi's specific question, "What does 'Buckle up' here [in the context of the news headline 'Buckle up: As Mueller probe enters second year, Trump and allies go on war footing'] exactly mean?" I second Louise's sensible view that it means readers of the headline should prepare for a bumpy metaphorical ride as opponents of the independent Mueller investigation into Russian interference in the 2016 U.S. elections seek to discredit and ultimately halt that investigation. The ride in question is the anticipated (and actually ongoing) series of legal and political maneuvers by the Trump administration and its allies (on the one hand) and by supporters of the investigation (on the other), even as the Mueller team itself maintains strict public silence about its progress, punctuated by occasional bursts of indictments.

In my view, juxtaposing the metaphors of "buckling up" (fastening a seat belt) and "going on a war footing" (marshaling one's forces in preparation for military conflict) isn't especially effective. But that's what the headline writer did. At least the headine doesn't conclude with "Pass the Popcorn."

Solution 3:

"Buckle up" (slang, as you note, for "prepare for a rough ride"), along with the comment about Trump going on a war footing, means, according to the headline writer, that there is going to be considerable conflict between the Mueller investigation of Trump and his political aides and the Trump White House.

"Going on a war footing" here is metaphorical, as is "buckle up". The possible outcome is not actual war, of course, but a Constitutional crisis.