In his speech to the American Society of Newspaper Editors Tuesday, President Obama called the budget proposed by Rep. Paul Ryan (R-Wis.) “thinly veiled social Darwinism.”

What does that mean?

Simply put, it means applying the ideas of Charles Darwin — that species adapt over time — to human society, arguing that competition over resources helps humanity evolve for the better as the weak are weeded out and the strong survive and thrive.

But social Darwinism is seen as more as an epithet than a useful description, because the idea is so malleable.

Social Darwinism gained some popularity early on among British landowners and American capitalists, who saw in it a justification for their own wealth.

Imperialism was justified on Darwinist grounds as strength honed through warfare, with stronger races overtaking weaker ones. But peace was too, on the grounds that the fittest members of society should not be wasted in war.

“The concentration of capital is a necessity for meeting the demands of our day, and as such should not be looked at askance, but be encouraged,” Carnegie wrote in his autobiography after reading social Darwinist theory. “There is nothing detrimental to human society in it, but much that is, or is bound soon to become, beneficial.”

Social Darwinism is also seen in eugenics, the idea that certain races and physical traits should be weeded out of the general population. It played a role in the American progressive movement and in Nazi Germany — both movements that went against laissez-faire capitalism, in very different ways. In this interpretation, the weak must be culled so that the society as a whole can evolve more quickly.

Thus “social Darwinism” can be used to attack all sorts of enemies.

Obama is arguing that Ryan, by radically transforming social welfare programs, would pit Americans against each other for resources and let the poor and weak die out — “dog eat dog” capitalism.

Social conservatives use the term to attack evolution as a scientific theory, arguing that Darwin’s ideas lead to race-based infanticide and euthanasia.

Critics argue that both eugenics and laissez-faire capitalism drew less from Darwin than popular history would suggest.

Sociologist Herbert Spencer, who coined the term “survival of the fittest” and popularized the idea that competition was good in both nature and society, began espousing his ideas before “The Origin of the Species” was even written. Many of his ideas on evolution came from other biologists and from Victorian culture, not Darwin. (Some now see Spencer as an early libertarian.)

Darwin himself did come to embrace social Darwinism, but he did not invent it.

While the term “social Darwinism” was used as early as 1877, it did not gain widespread popularity except in retrospect, and mostly as an insult.

So the best explanation probably comes from historian Robert Bannister: “Social Darwinism, as almost everyone knows, is a Bad Thing.”

Read more from PostPolitics

A ‘bromance’ blooms in Wisconsin

Sheriff Arpaio rejects court monitor; DOJ threatens to sue

The fat lady sings in Wisconsin

Fact Checker: Obama’s attack on the GOP budget