English
acorn nut
From acorn + nut, because it resembles an acorn.
Proto-Germanic *akraną ‘nut or mast, especially an acorn’
Proto-West Germanic *akarn ‘nut, mast, acorn’
Old English æcern
Middle English acorn ‘an acorn (oak fruit) or similar fruit’
English acorn
Proto-Italic *knuks
…
Proto-Germanic *hnuts
Proto-West Germanic *hnut
Old English hnutu
Middle English note
English nut
The story
From acorn + nut, because it resembles an acorn.
Today it means
noun A closed nut having a dome-like cap over the rear protruding part of the engaged screw or bolt.
Language periods are conventional ranges, not dates for this word.
Etymograph / acorn nut