"The soldier is a peculiar animal that can alone be brought to the highest effciency by inducing him to believe that he belongs to a regiment which is infinitely superior to the others around him." -Major-General Sir Garnet J. Wolseley (The Soldier's Pocket-book for Field Service, 1874)
"Keep your hands off the regiment, ye iconoclastic civilian officials who meddle and muddle in Army matters. Clever politicians you may be, but you are not soldiers and you do not understand them; they are not pawns on a chessboard. Leave the management of our fighting men to soldiers of experience in our British Army of old renown, and do not parody us by appearing in public decked for the nonce in a soldier's khaki coat. You might as well put your arm in a sling, or tie your head up in the bandage of some poor maimed soldier, to whom, when wounded and unable to earn a livelihood, your regulations allow a pension of sixpence a day!" -Field Marshal Viscount Wolseley (The Story of a Soldier's Life, Volume 2, 1903)
For starters, Field Marshal Garnet Joseph Wolseley, 1st Viscount Wolseley, KP, GCB, OM, GCMG, VD, PC was an officer of the British Army in the Victorian Era, definitely one of the most influential, highly regarded, and successful British generals of the second half of the 19th century, who was known for promoting efficiency with his role in modernizing the British Army. For 5 years (1895-1900), he held the position of Commander-in-Chief of the Forces, retiring in 1901. He was the model for the character Major-General Stanley in the operetta Pirates of Penzance by Gilbert and Sullivan (where "the very model of a modern major-general" derives from, the Major-General's Song).
Knowing the background of Lord Wolseley, you may conclude that the origin and definition of this idiom stems from his promotion of efficiency, which is a truth. The phrase is quite dated British slang, but it means that all is in order, everything's okay, or perfect (all synonyms). If everything's 'all Sir Garnet', then everything is all in order.
-Slang and Its Analogues Past and Present: A Dictionary ... with Synonyms in English, French ... Etc. Compiled by J.S. Farmer [and W.E. Henley], Volume 6 (1903)
"Flashman shows him briefly as a young staff-officer, before the full flowering of the quick temper and impatient efficiency which were to make the expression 'All Sir Garnet' synonymous with the modern 'Right on!' Wolseley always wanted the best; typically, he chose for one campaign a man who had beaten him in competition."
-The introduction to Flashman and the Dragon by George Macdonald Fraser, 1986
"Righto; that's all Sir Garnet. I like to see you civvies act up to your name."
-1905, Longman's Magazine, volume 46, page 152
"Yes, Harry was all Sir Garnet on handles: butt-end loaded with lead, inlaid in all sorts of fancy-work — hearts, shamrocks, monograms and that sort of thing."
-1907, The Lone Hand, volume 2, page 554
"'That's all Sir Garnet, and don't you make no mistake. Don't you — make no mistake.' Here Mrs. Frith gave a very loud hiccup and waved her arms and did not even say “beg pardon” for the offensive noise."
-1913, Compton Mackenzie, Youth's Encounter, Bell & Cockburn
“They're comin' along,” he cried to Mackenzie, as the thud of galloping horses was heard in the rear. “If I can only do the Horatius on the bridge business till they gits 'ere, we shall be all Sir Garnet.”
-Harmsworth Magazine, 1901.
I guess you can say...that everything truly is all Sir Garnet...HELP I HAVEN'T BEEN FED IN WEEKS AND
https://babel.hathitrust.org/cgi/pt?id=nyp.33433009290994&view=1up&seq=9&skin=2021
https://play.google.com/books/reader?id=nYtCbNpBt_wC&pg=GBS.PR2&hl=en
https://www.worldwidewords.org/weirdwords/ww-gar5.htm
https://en.wiktionary.org/wiki/all_Sir_Garnet
https://www.nam.ac.uk/explore/garnet-wolseley
https://collection.nam.ac.uk/detail.php?acc=2006-10-4-1
https://en.wikipedia.org/wiki/Garnet_Wolseley,_1st_Viscount_Wolseley
https://books.google.com/books?id=JKRBAAAAYAAJ&pg=PA215#v=onepage&q&f=false
https://victorianweb.org/history/crimea/beck/3.html