Green’s Dictionary of Slang

one who talks nonsense.

1913		D.H. Lawrence letter 17 Jan. Coll. Letters (1962) I 179: Don’t ever mind what I say. I am a great bosher, and full of fancies that interest me.
1939		H.G. Wells Holy Terror 89: ‘The man’s a blatant bosher.’ ‘He’s not. He knows exactly what he is up to.’ .