What defines a brew from acorns as a coffee as opposed to tea, as both take their names from their original ingredient? Are they ground up or something?
Though I am not old enough to recall the war years, I have experienced ersatz coffee made from acorns in German youth hostels of the 1960s. It is APPALLING - actually, no, it isn't that good. Avoid it like a thousand plagues!!
Acorn bread is not uncommon in parts of Europe, and acorn flour is available. The bread's not to everyone's liking, so wheat flour is often added to give a better taste.
And no, coffee can't made from acorns, but "coffee substitute" can be and is. (There's also 'dandelion coffee', made from the roasted and ground root).