All 4 Uses

solemn
in
It Happened to Nancy
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He told me that when his parents had said that they were going to divorce, right then and there he made a commitment, solemn as it could be, that he would never have sex until he was married.†
p. 76.6 *

solemn = very serious--possibly dignified
We've made solemn, sacred decisions and commitments, and we'll keep them.†
p. 81.8
RIGHT HERE IN WRITING, I'M MAKING A SOLEMN PROMISE THAT I WILL NEVER, EVER, EVER THINK OF *8c.†
p. 121.6
I hereby do solemnly promise with a blood oath....OHHHHH, THE PAIN.†
p. 121.8

solemnly = with seriousness and dignity

Definitions:

(1) (solemn)
in a very serious (and often dignified) manner
(2) (meaning too rare to warrant focus)
Less commonly, solemn can mean that something was done with great or appropriate ceremony. It can also be used to describe something as dark or undecorated.