Someone in a ruminative mood is thinking deeply and carefully, turning an idea over slowly, the way a cow turns grass over. It is a quiet, bookish adjective. You will meet it in novels and essays more than in speech: a ruminative pause, a ruminative look out of the window. It does not have to mean sad, though it often arrives with a slightly serious face. It is close to thoughtful, but slower and more inside the head — not planning a shopping list, more chewing on a memory or a hard question. In psychology, rumination can mean getting stuck on dark thoughts; ruminative in ordinary writing is usually milder than that. It is formal. Friends would just say lost in thought.