Nominalization of the 連用形(ren'yōkei, “continuative or stem form”) of verb 寿く(kotobuku), itself an alteration of earlier root form kotohoku, a compound of 言(koto, “words, speech”) + 祝く(hoku, “to pray for a good outcome”, obsolete in modern Japanese).[2]
The meaning shifted over time from to say something in prayer for a good outcome to a more general to say something favorable in hope for or upon a felicitous or auspiciousoccasion. From there, the meaning expanded to include long life.
This long life meaning was apparently already in use when the kanji was first imported and applied to this word.
Nominalization of the 連用形(ren'yōkei, “continuative or stem form”) of verb 寿ぐ(kotohogu), itself an alteration of earlier root form kotohoku, a compound of 言(koto, “words, speech”) + 祝く(hoku, “to pray for a good outcome”, obsolete in modern Japanese).[2]