Aside from some very marked contexts[1], "You ain't seen nothing yet", (and the equivalent using standard verb forms "You haven't seen nothing yet"), does not mean, and never has meant, "You have seen something" in any dialect of English. This is because English is not logic, it is a human language and used by humans, and humans like to pile on the negatives when they want to express something negative.
After somebody invented a rule that said you shouldn't say "not seen nothing", generations of pedants and pedagogues have made this ridiculous claim (that the negatives cancel out) in order to provide some rationalisation for the arbitrary rule.
"You ain't see nothing yet!" unambiguously says "you haven't seen anything yet", with the implication that "what you have seen is nothing in comparison with what is to come".
[1] One can concoct an example like "You think that's got nothing? No, you watch (name some film the speaker thinks is boring) and then you'll see nothing. You ain't seen nothing yet!" where the two negatives do cancel out. But this depends both on context and on particular intonation.
What does “in the name of…” actually mean?
Putting all religious contentions aside for the sake of our language, the etymology of name offers a good place to start understanding:
Old English nama, noma "name, reputation,"
from Proto-Germanic *namon
(cognates: Old Saxon namo, Old Frisian nama, Old High German namo,
German Name, Middle Dutch name, Dutch naam, Old Norse nafn, Gothic
namo "name"),
from PIE *nomn- (cognates: Sanskrit nama; Avestan nama; Greek onoma,
onyma; Latin nomen; Old Church Slavonic ime, genitive imene; Russian
imya; Old Irish ainm; Old Welsh anu "name").
Emphasis mine
We've all experienced the power of namedropping in our lives. People respect us and our opinions if they believe we are connected to someone with greater reputation and authority.
In all cultures, people of authority have always lent their reputation and their authority to their delegates. The founders and leaders of religious movements use the same delegation strategies as the founders and leaders of nations. The English phrase in the name of simply asserts the reputation and authority of another person.
English Reports Annotated - Pages 1505-2672, 1505, page 2048:
...an action on a board given to trustees of an industrial society
before the act may, after registration under the act, be brought in
the name of the newly -incorporated body.
Victor Hugo's Dramas 1519, page 364:
Richard Varney, in the name of God and Saint George we dub thee
knight!
The Newe Testament in Englishe Translated After the Greke, 1553:
And he that receiveth a righteous man, in the name of a righteous man,
shall receive a righteous man's reward.
Calendar of State Papers, Foreign Series, of the reign of Elizabeth: preserved in the State Paper Department of Her Majesty's Public Record Office, Volume 7, 1564:
Smith and Throckmorton in the name of their Mistress demanded the
ratification of the Treaty of Cateau Cambresis
An Open Letter to the Christian Nobility of the German Nation, by Martin Luther, 1520, Translation by C. M. Jacobs, Page 94:
The complaint was made at Worms (1521) that it was impossible for a
German to secure a clear title to a benefice at Rome unless he applied
for it in the name of an Italian, to whom he was obliged to pay a
percentage of the income...
Emphasis mine
We introduce an interrogative with the emphatic: What in God's name, or its metonym: What in heaven's name. That emphasis poses an implication to the listener: I have a right to ask this question, and you owe me an answer!
Best Answer
I agree in this context with Spencer's comment that place and time are orthogonal concepts; however, this mathematical concept may be out of scope of an ELU discussion (I may be wrong though).
Let's start with a dictionary definition.
Cambridge Dictionary:
The idiomatic definition is that someone in such a situation just got unlucky and this proves that your assumption is wrong.
Now, let's try to decompose it literally using an example of a cat wanting to take a nap. If the cat chooses a fireplace to do so, it is a wrong place in general; however, it may not be so when there is no fire therein and it is at near room temperature. Now, suppose someone decides to light up a fire there (and is somehow oblivious of the cat's presence). Lo and behold, it now also becomes the wrong time! Now, both negatives add up to more negative!
Also, consider the example accompanying the definition above: it mentions that one can get hurt if they are at the wrong place (at a place through which the eye of the storm passes) at the wrong time (when it does so).