Fall on your sword
Commit suicide or offer your resignation.
It's been some time since men routinely carried swords and the use of 'falling on one's sword' is now restricted to the figurative usage when someone takes personal responsibility for a group action. The expression was used widely following the resignation of Lord Peter Carrington, who resigned from his post as Foreign Secretary for the Thatcher government in 1982, following Argentina's invasion of the Falkland Islands. He was the last high-profile politician in the UK to take personal responsibility in such circumstances.
The actual practise of committing suicide by falling on one's sword dates back to ancient Rome. Plutarch records such a death in The Life of Brutus:
Finally, he [Brutus] spoke to Volumnius himself in Greek, reminding him of their student life, and begged him to grasp his sword with him and help him drive home the blow. And when Volumnius refused, and the rest likewise ... grasping with both hands the hilt of his naked sword, he fell upon it and died.
The above account was published in English in 1918.
The notion was already current in English in the 16th century. It appears in The Miles Coverdale Bible, 1535, in an account of the death of Saul - Samuel 31:4-5:
Then sayde Saul vnto his wapebearer: Drawe out thy swerde, and thrust it thorow me, that these vncircumcised come not and slaie me, and make a laughinge stocke of me. Neuertheles his wapenbearer wolde not, for he was sore afrayed. Then toke Saul ye swerde, and fell therin.
Now whan his wapenbearer sawe that Saul was deed, he fell also vpon his swerde, and dyed with him.
Shakespeare alludes to a similar scene, in the death of Mark Antony, in Julius Caesar, 1601, although he didn't use a version of the 'falling on one's sword' text.
The expression is the Anglicized equivalent of hara-kiri - the Japanese samurai custom of committing suicide by disembowelment with a sword rather than face the dishonour of surrender. The highly ritualised and formal hara-kiri suicide - literally 'belly cut', is no longer performed. It has been known about in the West since the mid 19th century and was referred to in 1856 in Harper's Magazine in the title of an article - Hari-kari of Japan. It that piece Harper's used, and possibly originated, the common misspelling 'hari-kari'.