This word is now rare to the point of complete disuse. It comes from Greek leios, smooth, plus trikhos, hair, hence having straight hair.
That it exists at all is due to the French naturalist Baron Jean Baptise Genevieve Marcellin Bory de Saint-Vincent, who travelled the world at the beginning of the nineteenth century studying plants. He also made a stab at classifying peoples into races. He is now hardly remembered, but in a once-influential book Homo: essai zoologique sur le genre humain, published in Paris in 1827, he attempted to classify humans with straight and wavy hair into the Leiotrichi and those with woolly or tufted hair into the Ulotrichi, with many sub-groups below these headings.
His classification was seriously studied for several decades, being quoted — for example — by Thomas Henry Huxley and Charles Darwin. The adjective ulotrichous (Greek oulos, woolly), from his other main category is also rare, but the related lissotrichous, smooth-haired, is still in the vocabulary of some specialists, especially zoologists; this comes from Greek lissos, which also means smooth. A third category is that of wavy-haired or cymotrichous people (from Greek kuma, wave). These last three adjectives have been used to classify types of hair, for example in forensic identification.