Indefinably is how you say the same idea when you are describing the manner of it. If a song is indefinably sad, the sadness is there, but you cannot say in what way, or why it sits in you. It almost always stands in front of an adjective: indefinably strange, indefinably familiar, indefinably different. The job of the word is to admit a limit: you can point at the quality, you cannot unpack it. Same slightly bookish feel as indefinable — a little literary, a little careful. In speech, many people would just say "I don't know why, but…" and skip this word.

難以名狀地、說不上來地