Literally translated, ofermode means "over-heart" or "over-spirit" in Old English. However, scholars have debated whether the word connotes confident courage or foolhardy pride. I think this ambiguity is awesome and I hope it is never resolved.
Where it comes from:
The most famous instance of ofermode comes from a fragmentary Anglo-Saxon poem called "The Battle of Maldon" which celebrates the glorious defeat of the English at the hands of the Vikings. The poem is written from the perspective of the English and it recounts one of the most wonderful, blunderful military decisions of all time.
The fragment begins with Byrhtnoth, the leader of the English, marshaling his warriors to face the band of Viking pirates who have been raiding the coast of Essex. The Vikings are numerous, but they are at a disadvantage because Byrhtnoth's men are fighting them at a natural choke point. The Vikings had beached their ships on a small island just offshore that connects to the mainland only by a narrow causeway at low tide.
As Byrhtnoth positions his warriors on the mainland and appears ready to kick some ass if the Vikings dare to cross, a Viking messenger shouts across the channel to offer a truce for tribute, saying, "No need to slaughter each other if you be generous with us; / we would be willing for gold to bring a truce." Byrhtnoth, of course, refuses them in the most manly and impressive way possible. He says (among other manly and impressive things), "fierce conflict is the tribute we will hand over."
So when the tide goes down and the Vikings cross the causeway, the English begin to slaughter them one by one. Seeing his men get cut down with no room to maneuver, the Viking leader shouts across the channel and demands a fair fight—that is, he wants nothing less than a timeout for his army to position itself on even terrain before the battle resumes. Would Byrhtnoth really give up a strategic advantage and endanger his homeland for the chance of winning greater glory on the battlefield?
Yeah, of course he would:
Then the Earl permitted in his great pride
to allow land many of these hateful people;
and so then shouted on the shore of the cold water
Byrhtelm's child -- and the warriors listened:
"Now the way is open to you: come quickly to us
you men to battle. God alone knows
who on this field of honor may be allowed to be the master."
Then advanced the wolves of slaughter...
As you might expect, Byrhtnoth and his men are completely overwhelmed. Byrhtnoth himself is cut down in a suicidal stand against a number of foes, but only after he kills three Vikings and gets his sword-arm hacked up. The men of Essex die with their liege lord (though some cowards flee), and the tribute money is paid to the Vikings shortly after Byrhtnoth's men die. Their heroic stand proves as futile as it was unnecessary.
But the strange thing is this: the poet of Maldon might not be critical of Byrhtnoth when he attributes the decision to his ofermode. The poet decidedly condemns the English who flee the battle and do not match their boasting words with heroic action. The poet also seems critical of the Vikings who complain and use guile to gain the upper hand in battle. But Byrhtnoth is perhaps honored for his heroism: he directly and decisively confronts the enemy, letting his own strength and courage decide the outcome of battle. His straightforward heroism is a direct counter to the Vikings' reliance on hit and run tactics, opportunism, and negotiation. By meeting the Vikings on the open battlefield, he denies them the opportunity of retreating to their ships to attack Essex again or to strike elsewhere along the coast.
Or, at least he thinks that's what he's doing. If ofermode can mean both courage and arrogance then we can celebrate Byrhtnoth for his heroic folly, a quality that immortalizes him because it allows his actions transcend defeat and victory.