The word "well-marked" implies clarity, distinctiveness, and obviousness. Its antonyms are words that denote vagueness, obscurity, and ambiguity. Some of the antonyms for "well-marked" include indistinct, unclear, vague, fuzzy, misty, blurry, hazy, and diffuse. These words indicate a lack of clear boundaries, making it difficult to identify or distinguish the object being described. For example, an unclear map is not well-marked and can cause confusion for its users. In contrast, a well-marked path in a forest is easy to follow and provides clear guidance. By using the antonyms of "well-marked," we can express a range of meanings that indicate a lack of clarity or definition.