bite the bullet 「歯を食いしばってがんばる」

このところ困難に立ち向かう男らしさ（あるいは、つらさ）にちなんだ表現を取り上げたが、もうひとつ、bite the bullet について短く書きたい。麻酔なしで手術する際に患者に弾丸を噛ませたことに由来するというので、何ともリアルな感じがする。

- to force yourself to do something unpleasant or difficult, or to be brave in a difficult situation: I hate going to the dentist, but I suppose I'll just have to bite the bullet.
(Cambridge Advanced Learner's Dictionary & Thesaurus)

- Decide to do something difficult or unpleasant that one has been putting off or hesitating over.
But now I have decided to bite the bullet and give it a go.
[from the old custom of giving wounded soldiers a bullet to bite on when undergoing surgery without anesthetic]
(Oxford Dictionaries)

- (idiomatic) To endure a punishment or consequence with dignity or stoicism.
(idiomatic) To accept a negative aspect of a situation in order to proceed.
Etymology
Possibly from the reported practice of soldiers biting a bullet to avoid crying out in pain, usually during a medical procedure or punishment.
(Wiktionary)

ところで「麻酔の代わりに弾丸をくわえさせたのが由来」という説は本当なのか。The Phrase Finder というサイトは、

- Is it true or is it just 'believed'? The theory goes that patients undergoing surgery would be given a stick of wood or a pad of leather to bite on in order to concentrate their attention away from the pain and also to protect against biting their own tongues. A bullet, being somewhat malleable and not likely to break the patient's teeth, is said to have been an impromptu battlefield alternative. Lead poisoning would probably have been a secondary concern in those circumstances.
( ttp://www.phrases.org.uk/meanings/bite-the-bullet.html )

と書いているが、そのほかにもおもしろい情報が載っているので、興味がある方はご覧になってみてはいかがと思う。

なお bite や bullet については、以前、下記のような表現を取り上げたことがあるので、参考までに記しておきたい。
（→ 一敗地にまみれる bite the dust）
（→「特効薬」あれこれ (magic bullet, silver bullet)）
（→ bullet point （・） 「黒マル、中点、中黒、中ポツ」「箇条書き」）