Acanthoid refers to something that resembles or has spines like a thorn or a spine. The antonyms for acanthoid could be smooth or spineless. Smooth refers to something free from unevenness or roughness, which is the opposite of acanthoid. Spineless means without a spine or thorns, which is the opposite of a spiky or thorny acanthoid. Other antonyms for acanthoid can be soft, gentle, or rounded. These antonyms depict a different structure or character from acanthoid, which portrays sharpness and pointedness. Thus, depending on the context, antonyms for acanthoid could be smooth, spineless, soft, rounded, or gentle.