I drank of the jar.
Is this 'of' an old usage?
But I can't find the proof for that.
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This tends to be an older usage.
It's not hard proof, but you will find a comparison of different versions of the same bible verse here of varying degrees of antiquity.
They compare a line in the book of Genesis
And he drank of the wine, and was drunken; and he was uncovered within his tent."
-King James Bible published 1611
Many of the newer versions of the bible use the line:
He drank some of the wine, became drunk, and uncovered himself inside his tent.
-New International Version published 1978 and revised since.
This usage of of relates to the stuff that is consumed, not the container -- the "jar" I suppose symbolically represents its contents here (the context should make it clear).
The closest dictionary definition of contemporary use is:
of 7 Indicating the relationship between a verb and an indirect object
You can find its use in literature.
Sacred hot springs making a comeback in Europe, NPR_Sunday, 2002:
Mr-MOLDOVEANA: These waters were mysterious. If you drank of normal waters, which was transparent and fresh, and all of a sudden, you will find a water which is reddish or greenish, boiling, foul-smelling, you will be inclined to think that this is monstrous water, this is a force directed against you, against your well-being.
The swamp of dreams, Harper's Magazine, 1993:
After eating these, he drank of the cool water of a shallow brook, then washed his face, further clearing his mind.
Warriors of Christendom, 1988:
… he took of that balsam and of that myrrh as much as a little spoon-full, and mingled it in the cup with rose-water and drank of it; …
War in high heels, 1993:
Later, Jacqui met her kiss for kiss in their lovemaking; even taking the initiative as they drank of each other 's bodies.