UPDATE
February 01, 2018, the OED has recently added the word swag in its dictionary. Oxford Online Dictionaries reports
A new entry has been added for swag, derived from swagger, and used in slang to denote ‘bold self-assurance in style or manner’, or ‘an air of great self-confidence or superiority’. The OED’s first citation for this particular sense comes from the track ‘December 4th’ on Jay-Z’s The Black Album (2003): ‘My self-esteem went through the roof, man. I got my swag.’ This is the fifth OED citation attributed to Jay-Z.
A glossarial example of the word from the previous year, in a self-described dictionary of hip-hop terminology, defined swag as simply ‘walk’.
ORIGINAL POST
(October 12, 2016)
“Obama displayed similar swag and bluster …”
In his sixth State of the Union Address, President Obama said:
We’re upholding the principle that bigger nations can’t bully the small — by opposing Russian aggression, supporting Ukraine’s democracy and reassuring our NATO allies. Last year, as we were doing the hard work of imposing sanctions along with our allies, some suggested that Mr. Putin’s aggression was a masterful display of strategy and strength. Well, today, it is America that stands strong and united with our allies, while Russia is isolated, with its economy in tatters. That’s how America leads — not with bluster, but with persistent, steady resolve (Applause).
From whitehouse.gov, January 20, 2015
The swag appears to be a clipped form of swagger, which the article accuses President Obama of doing. In the speech, the ‘swag’ refers to America standing strong, unafraid of Russia, and determined to enforce sanctions. The bluster, according to the author, is Obama's words whose tone was possibly concealing a threat to Putin. The fact that Obama asserted America did not engage in bluster, was telling the audience that America was not afraid of taking further action if necessary.
bluster
talk intended to seem important or threatening but which is not taken
seriously and has little effect
swagger
to walk, esp. with a swinging movement, in a way that
shows that you are confident and think you are important
Cambridge Dictionary
If a thing is slap-bang on, it would mean "exactly right". His answer to the quiz was slap-bang on.
That said, one would not usually refer to a person as being: slap-bang on.
Please note: the term is slap-bang [right, as in right in the middle of] and usually referring to a THING, not a PERSON. Slap-bang is used to intensify: the dog was slap-bang in the middle of the road.
The other term: to be bang on means to be right. slap-bang on merely makes it stronger.
So, if you want to join /bang on/ the verb to /slap-bang/ the adjective, the best way would be: slap-bang on.
That said, it is very odd to say that: Princess Diana was slap bang-on. The only meaning here would be: She was right about the royal unspoken code of marriage.
That means: She knew that if Charles could have Camilla, then she Diana was perfectly justified (slap-bang on or right about) about having that lover of hers (I forget this name).
Best Answer
The nearest meaning is crash after a drug high [link]
In the linked post, the original poster says:
This is essentially euphoric - like euphoric high similar to that from a drug. The answer says this will end soon, and end badly, with a very bad depressive feeling - "a crash", and the asker will need help of friends in making the actual recovery.