Wriggling or winding in a snakelike, sinuous manner; twisting or meandering with curves (onomatopoeic).
ヘビが草の間をうねうねと進んでいった。
A snake slid sinuously through the grass.
湿気で髪がうねうねしてまとまらない。
My hair gets all wavy and unruly because of the humidity.
山道はうねうねと曲がりくねっていた。
The mountain road twisted and turned in winding curves.