Blunder means a mistake or error. He made a huge blunder when he invested in a useless stock.
A serious thoughtless mistake
Blunder is a serious error committed due to lack of knowledge, confusion or panic. The consequences of a blunder can be quite magnanimous. History has witnessed a lot of blunders which has changed the course of events in the past. Some of the examples of blunders would be the attack on Pearl Harbour by Japan, the Boston Tea Party of 1773, and the Kargil War in 1999. Blunder commonly happens when one acts due to flawed perception or misinterprets the possibility of events.
The word blunder comes from the Norse (ancient German language) "blunda" which means "to shut one's eyes or to become oblivious." Blunders can even be a blessing in disguise. For example Edison has been quoted as saying that he has successfully learnt things which do not work. In chess, a blunder means a bad move. A blunder with less disastrous consequences can be called a blooper or a faux pas. Here is a sentence which makes the meaning of blunder clear: "The skipper committed a big blunder by asking his spin bowler to bowl the last few overs of the match."