The word "坎坷" is formed by combining two characters, each containing the "土" (earth) radical, linking them semantically to the ground. "坎" means a pit or uneven area, while "坷" similarly denotes roughness or obstructions on the ground. Together, they create a compound through semantic repetition, where both characters contribute nearly identical meanings of uneven terrain, literally referring to a bumpy path and by extension metaphorically to hardships.