Why "保重" means "take care"?
It is true that this word originally comes from 《晋书·夏侯湛传》：“方将保重啬神，独善其身。”
But we use it in situation of farewell. 珍重 is heavier than 保重。 珍重包含了珍惜cherish.

保 是保住，保持，守 。 keep， reserve probably.

重 has two tones:


zhòng 是分量较大，与“轻”相反。 large amount or heavy. opposite with light. 很重，举重。
chóng means again. doubled. 重复，重播。


In the phrase 保重 is zhòng. In my appoint: strong power. strength.

Does it make sense? (Reference.)