From Old Norse 'blunda' (to shut the eyes) — a blunder is the mistake of someone moving with eyes closed, stumbling blindly.
A stupid or careless mistake.
From Middle English 'blunderen' (to stumble, to move blindly), probably from Old Norse 'blunda' (to shut one's eyes, to doze, to nap), from Proto-Germanic '*blundaz' (blind, dim-sighted). A blunder is literally the mistake of someone moving with their eyes shut — stumbling blindly. The word may be related to 'blind' and 'blend' (which originally meant 'to mix up, to confuse'), all from the idea of impaired vision leading to confused action. Key roots: *blundaz (Proto-Germanic: "blind, dim-sighted").
In Scandinavian languages, 'blunda' still means 'to close one's eyes' — Swedish 'blunda' and Norwegian 'blunde' carry this gentle meaning of simply shutting your eyes. English took the Norse word and made it clumsy: to 'blunder' is to stumble around as if your eyes were closed. Chess adopted the term — a 'blunder' is a catastrophically bad move, and in chess notation it is marked with '??', the only sport that has standardized punctuation for stupidity.