The English phrase “blood, sweat and tears” refers to hard work or effort that is put into a project. This colorful idiom combines these various bodily fluids in a sometimes metaphorical way, where many successful projects will not require the kinds of injuries associated with blood. Likewise, the use of “tears” or even "sweat" are often metaphorical. Using this idiom in conversation illustrates an intense investment of effort, and may also imply a deep personal attachment to the final product of the effort. The phrase is also the name of a now-famous rock band from the 1980s.
According to most historians, the phrase “blood, sweat and tears” came from a speech by British leader Winston Churchill in 1940, while that nation was engaged in the dire battle for domination during the era of World War II. Churchill’s exact words according to record were: “I have nothing to offer but blood, toil, tears and sweat” where the words “tears” and “sweat” are inverted from the now-common use and the word “toil” is added. It’s likely that “toil” was not included in the common usage because it does not represent the same physical essence as the other three items, and it also makes it more cumbersome to say.
Those who are addressing the idea of putting a lot of effort into a project might use a similar phrase to illustrate hope that effort is associated with a rightful ownership. Here, someone might say that if someone put a lot of work into something, they have some “sweat equity.” The word equity refers to value that a person puts into something in order to get value back out at a later time.
With the sorts of complicated modern financial agreements in place in today’s financial world, the idea of sweat equity, or someone owning a project by providing “blood, sweat and tears,” is often no longer a practical one. For example, many startup businesses sign debt agreements with outside investors who give money to the budding business in exchange for future control through its stock. Here, sweat equity will not guarantee future control.
In modern usage, “blood, sweat and tears” commonly refers to effort that may have been “invested” in vain or without material compensation. Some may talk about non-profit projects or non-revenue generating intellectual projects as having been the recipients of “blood, sweat and tears.” The phrase generally implies a great deal of ingenuity and pride on the part of the one who initiated a project, as well as diligence and hard work.