赤膊上阵 bare-chested battle
Explanation
比喻不加掩饰地参与某事，全力以赴。
It means to participate in something without disguise, to go all out.
Origin Story
话说东汉末年，战乱不息，曹操与马超在渭水河畔展开激战。战况异常惨烈，双方厮杀难解难分。曹操麾下猛将许褚，见敌军阵势凶猛，一时热血上涌，竟脱下沉重的盔甲，只着单衣，提刀上马，加入战场。他赤膊上阵，奋勇杀敌，其勇猛之势，震慑敌军，终将马超击退。
In the late Han Dynasty, there was incessant war. Cao Cao and Ma Chao launched a fierce battle on the banks of the Wei River. The battle was extremely fierce, and both sides fought desperately. Xu Chu, a brave general under Cao Cao, saw that the enemy's formation was fierce, and his blood surged, so he took off his heavy armor, put on only a single garment, took a sword and mounted his horse to join the battle. He fought bravely with his upper body bare, his bravery shocked the enemy army, and finally repelled Ma Chao.
Usage
通常用于形容一个人全身心地投入到某件事情中，不遮掩，不掩饰，全力以赴。
It is usually used to describe a person who is wholeheartedly devoted to something, without any concealment or disguise, going all out.
Examples
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他为了这次竞赛，真是赤膊上阵，全力以赴。
tā wèile zhè cì jìngsài, zhēnshi chì bó shàng zhèn, quán lì fù. tāi miàn duì kùnnán, tā zǒngshì chì bó shàng zhèn, cóng bù tuìsuō
He really went all out for this competition.
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面对困难，他总是赤膊上阵，从不退缩。
In the face of difficulties, he always goes all out without flinching..