1. What does it mean?
솔까말 shortens 솔직히 까놓고 말해서. It announces that the speaker is about to give an unvarnished opinion, often when admitting something or disagreeing with the mood of a conversation.
Also written or said as 솔직히 까놓고 말해서
2. The nuance
The phrase can sound more blunt than a neutral 솔직히 말하면. It is not a free pass to be insulting. In a professional message, spell out the point politely instead of assuming this shorthand will sound friendly.
3. Examples
“솔까말, 그 영화는 예고편이 더 재미있었어.”
Honestly, I enjoyed the trailer more than the movie.
“솔까말 나도 처음에는 설명을 잘 못 알아들었어.”
To be honest, I didn't really understand the explanation at first either.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.