The idiom "鞠躬尽瘁" is formed by directly combining two two-character phrases: "鞠躬", which literally means to bow the body and signifies earnest effort, and "尽瘁", which means to exhaust oneself to the point of weariness. It originated from a historical document where these phrases appeared consecutively to describe total dedication, and through frequent usage, they coalesced into a fixed four-character compound denoting unwavering commitment until complete exhaustion.