blessing, happiness, prosperity
祜 centers on divine favor that brings well-being: a blessing that results in happiness and prosperity.
祜 combines 示 (altar, divine) with 古 (old), suggesting an ancient or long-standing blessing. The exact historical development is uncertain, but the phono-semantic structure is plausible.
At the altar 示, an old 古 tradition brings a blessing of happiness and prosperity.
For コ, imagine an old 古 altar 示 where a child 子 (こ) receives a blessing: 子 -> コ, and the child is filled with happiness.