To understand the line "Nature without check with original energy" in "Song of Myself" by Walt Whitman, it is important to perceive the speaker's intention in the poem. "Song of Myself" is a celebration of life. The opening lines proclaim, "I celebrate myself, and sing myself." Later, in the twenty-fourth section, the speaker identifies himself as "Walt Whitman, a kosmos." Kosmos is an archaic spelling of the word cosmos, which, according to Merriam-Webster, is defined as the following:
An orderly harmonious systematic universe; or, a complex orderly self-inclusive system.
In the forty-fourth section, the speaker adds, "What is known I strip away, / I launch all men and women forward with me into the Unknown."
Returning to the first section, we read that the speaker states that he is now making a beginning at "thirty-seven years old." What he means by this is that he is in a sense reborn to a new life. He desires to strip away the superfluities of the culture in which he has been brought up and to make a new start. In doing this, he has got back in touch with his natural and primeval instincts and energies. The poem is a declaration that he has succeeded in reaching this goal.
Just before the line in question, the speaker asserts that "creeds and schools" are "in abeyance." To be in abeyance means to be temporarily suspended. In other words, he has managed to strip away, temporarily at least, the effects of the dogmas, rules, strictures, and mores that he has been taught, and he is able to see the world through fresh eyes. Once he has accomplished this, he finds new strength and inspiration. This is the "original energy" in the line in question. Formerly his true human nature was held in check by the things he learned in "creeds and schools," but now that he has cast these norms of society aside, he is able to experience the energy of his true nature "without check."