rash, pain, debilitation
癆 centers on a state of physical distress: a rash or skin eruption, pain, and the resulting weakness or debilitation. The meanings are linked by the idea of a bodily affliction that causes suffering and drains strength.
癆 combines 疒 (sickness) with 勞 (labor, toil), suggesting a disease associated with exhaustion or overwork. The exact historical development is uncertain, but the phono-semantic structure is plausible.
Under the sickness roof 疒, the toiling component 勞 shows a person worn down by hard work. Imagine the body breaking out in a painful rash from exhaustion, leaving you weak and debilitated.
For ロウ, picture a laborer with a painful rash groaning 'low' in exhaustion: low -> ロウ, and the sickness 疒 saps their strength.
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tabes dorsalis; locomotor ataxia