The phrase translates a Sanskrit term from the tantric traditions of South Asia, where a distinction is drawn between practice conducted through orthodox and purifying means and practice conducted through substances and acts that ordinary observance treats as polluting. The second path is understood by its own practitioners as more dangerous and faster, undertaken under a qualified teacher, and its transgression is ritual rather than moral: the point is that the distinction between pure and impure is provisional, not that wrongdoing is good.
Western occultism took the term in the late nineteenth century, largely through Theosophical writing, and changed what it meant on the way. In its new setting it acquired a straightforwardly ethical sense, standing for selfish or malevolent magic set against benevolent magic, and that reading has been the dominant one in English ever since.
Twentieth-century writers then adopted the label deliberately as a self-description. The modern groups that call themselves left-hand path generally define themselves by the pursuit of individual apotheosis rather than union with a deity, and by a rejection of moral authority external to the practitioner. That is a coherent position and it is a modern Western one.
Three distinct things therefore share a name: a technical distinction within Hindu and Buddhist tantra, a nineteenth-century Western mistranslation of it, and a contemporary religious identity built on the mistranslation. The atlas keeps them separate.