know whereof one speaks

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know whereof one speaks

to know well the subject that one is speaking about. You are wrong! You do not know whereof you speak. He simply doesn't know whereof he speaks.
See also: know, one, speak
References in classic literature ?
When the people observed I was quiet, they discharged no more arrows; but, by the noise I heard, I knew their numbers increased; and about four yards from me, over against my right ear, I heard a knocking for above an hour, like that of people at work; when turning my head that way, as well as the pegs and strings would permit me, I saw a stage erected about a foot and a half from the ground, capable of holding four of the inhabitants, with two or three ladders to mount it: from whence one of them, who seemed to be a person of quality, made me a long speech, whereof I understood not one syllable.
He appeared to be of a middle age, and taller than any of the other three who attended him, whereof one was a page that held up his train, and seemed to be somewhat longer than my middle finger; the other two stood one on each side to support him.
He often builds his largest men of war, whereof some are nine feet long, in the woods where the timber grows, and has them carried on these engines three or four hundred yards to the sea.
In Witness whereof we have hereunto subscribed our names at Cape Cod the eleventh of November, in the Raigne of our Sovereigne Lord, King James of England, France, and Ireland, the eighteenth, and of Scotland, the fiftie-fourth, Anno.
BE IT REMEMBERED, That on the thirteenth day of June, in the forty-seventh year of the Independence of the United States of America, Charles Wiley, of the said District, hath deposited in this office the title of a Book, the right whereof he claims as proprietor, in the words and figures following, to wit:
You must except, nevertheless, Marcus Antonius, the half partner of the empire of Rome, and Appius Claudius, the decemvir and lawgiver; whereof the former was indeed a voluptuous man, and inordinate; but the latter was an austere and wise man: and therefore it seems (though rarely) that love can find entrance, not only into an open heart, but also into a heart well fortified, if watch be not well kept.
In testimony whereof, we have hereunto set our hands and seals beside those of the persons hereinabove named, for the information of all whom it may concern.
He was a tall man of middle age, with two goggle eyes whereof one was a fixture, a rubicund nose, a cadaverous face, and a suit of clothes (if the term be allowable when they suited him not at all) much the worse for wear, very much too small, and placed upon such a short allowance of buttons that it was marvellous how he contrived to keep them on.
Several very exciting skirmishes were in progress, when a loud shout attracted the attention even of the belligerents, and then there poured on to the platform, from a door at the side, a long line of gentlemen with their hats off, all looking behind them, and uttering vociferous cheers; the cause whereof was sufficiently explained when Sir Matthew Pupker and the two other real members of Parliament came to the front, amidst deafening shouts, and testified to each other in dumb motions that they had never seen such a glorious sight as that, in the whole course of thier public career.
And when the petition had been read and was about to be adopted, there came forward the Irish member (who was a young gentleman of ardent temperament,) with such a speech as only an Irish member can make, breathing the true soul and spirit of poetry, and poured forth with such fervour, that it made one warm to look at him; in the course whereof, he told them how he would demand the extension of that great boon to his native country; how he would claim for her equal rights in the muffin laws as in all other laws; and how he yet hoped to see the day when crumpets should be toasted in her lowly cabins, and muffin bells should ring in her rich green valleys.
She then proceeded thus: "I don't pretend to give your la'ship advice, whereof your la'ship knows much better than I can pretend to, being but a servant; but, i-fackins
Announcement of competition: pipetting robot is intended to be used for standardized and advanced setup of PCR, whereof pipetting DNA Mastermix in single tubes, 8-well strips and 96/384 wells plates.
And 'tho sometimes to his neighbours His tales became a joke, All his buddies listened quietly For they knew whereof he spoke.
By a great drought which continued from the third week in May, till about the middle of July, without any rain and with great heat for the most part, insomuch as the corn began to wither away though it was set with fish, the moisture whereof helped it much.
Each student would get two hundred thousand each in four equal annual instalments, first installment whereof were distributed in the ceremony.