thickthickthickerthickestadjective (thicker, thickest)1far from one side to the other: The walls are very thick. It's cold outside, so wear a thick coat.oppositethin2You use thick to say or ask how far something is from one side to the other: The ice is six centimetres thick.3with a lot of people or things close together: thick dark hair4If a liquid is thick, it does not flow easily: This paint is too thick.oppositethin5difficult to see through: thick smoke▶ thicknessthicknessthicknessesnoun (no plural): The wood is 3 cm in thickness.