Chiffy
"Etymologicon Magnum, or Universal Etymological Dictionary" by Walter Whiter (1800) makes the claim that "chiffy", as used in the term "in a chiffy" derives from the Anglo-Saxon word "Caf".
"A Dictionary of the Anglo-Saxon Language" by Joseph Bosworth (1832)
confirms the meaning of "Caf" as "quick, sharp, nimble, swift".
Jiffin
This is my oldest source yet, this time for "jiffin".
"The Fall of British Tyranny: or American Liberty Triumphant" by John Leacock
This was published in MDCCLXXVI, which by my reckoning is 1776.
Please to walk aft, brother soldiers, that's the fittest berth for you, the Kidnapper's in the state room, he'll hoist his sheet-anchor presently, he'll be up in a jiffin --- as soon as he has made fast his end of his small rope athwart Jenny Bluegarter and Kate Common's stern ports."
Jiff/Jiffy
In 1791, Edward Nairne of Sandwich, Kent published "Poems, Miscellaneous and Humorous, with Explanatory Notes and Observations" in which the following lines appear:
At dinner-time, and bus'ness slack,
I stept to Joe's, and got a snack
A pot of mildchee, and a whiff,
And off again in half a jiff !§
The author's explanatory notes, below, are expansive and delightful:
§ Jiff or jiffy, a jocular expression, and means a short space of
time. Innumerable are the expressions (particularly amongst sailors)
to shew what expedition may be, or is intended to be made, in the
doing of any act ; the progress of these is curious. I perfectly recol-
lect, when a school-boy, an expression of this kind — ' Before you
can say Jack Robinson' — was very common. After the intervention
of various others, that of — ' As soon as you can say peas' — came
into vogue ; but some persons, who were not over precipitate, very
properly qualified it by adding — ' and boil them.' Next, the ele-
gant expression of doing any thing ' In a pig's whisper' came into
fashion! (What particular period of time this contains, I am at a loss
to determine, having never yet had the pleasure of hearing these melodious animals exhibit in this way ! — I have frequently, and with
admiration, observed them make transitions from one note to another,
and which usually has a most charming effect.) — The ingenuity of
modern times has, I believe, brought this business to its ne plus ultra,
its greatest perfection ! and people can now, according to their own
declarations, do things ' In less than no time ' This beats Joshua's
making the sun stand still -, for that only protracted daylight, and
puzzled the clocksmiths ! but this has all the advantages of time,
without the inconvenience of waiting for it.
Best Answer
Popular definitions of 'dwankie'/'dwanky'
With regard to dwankie, a glossary of South African slang appended to Patricia Rice, Twin Genius: Family Genius Mystery #4 (2016) includes the brief entry:
Urban Dictionary offers this submission for dwankie from tokoloshe, posted on February 15, 2012:
And the same online resources has this submission for dwanky from StarKiller, posted on August 21, 2012:
Dictionary entries for 'dwang'/'in the dwang' and some Google Books matches
One older reference work gives a possible Afrikaans etymology for a similar-sounding slang term, dwang, used in the slang phrase "in the dwang." Here is the entry for that term in Jonathon Green, Chambers Slang Dictionary (2008):
Eric Partridge & Paul Beale, A Dictionary of Slang and Unconventional English, eighth edition (1984) has this entry for the same phrase:
Tom Dalzell & Terry Victor, The Concise New Partridge Dictionary of Slang and Unconventional English (2014) offers yet another possible source of the word dwang:
The earliest Google Books match for the phrase is from Andy De Klerk, In Search of the Strange: And Other Tales of South Africans at Large (2004) [combined snippets]:
But Tom Dalzell & Terry Victor, The New Partridge Dictionary of Slang and Unconventional English, second edition (2015) includes citations from 1988 (for the New Zealand wall stud support sense of dwang):
and from 2001 (for the longer phrase "in the dwang"):
Fuller's book is subtitled "An African Childhood," and her use of "in the dwang" appears in a remembered conversation between her and her fourteen-year-old sister in the late 1980s in Zambia, though earlier in her life she lived in Rhodesia (Zimbabwe), and her parents were British.
Where did 'dwankie'/'dwanky' come from?
The simplest possibility is that dwanky/dwankie arose from in the dwang in South Africa. If you think of the former as meaning "crappy" and the latter as meaning "in deep crap," the connection seems tolerably strong.
But other sources find the expression in use earlier than any mention of it appears in a Google Books match connected in any way to South Africa (or Zambia). And even if G.D. Wilson's memory of "in the dwang" from 1939 as conveyed to Eric Partridge in 1979 isn't trustworthy, we still have Partridge's inclusion of "in the dwang" in his 1984 dictionary.
So dwankie/dwangy could have emerged from "in the dwang" in South Africa around 2012 and yet still not indisputably have come from an Afrikaans root word. In the absence of clearer evidence of its roots than I've been able to find, I would list its etymology as uncertain.