-A past instance of active thought, to deliberate upon a problem or a solution.
-The action of thinking.
-The active analytic process of thinking and organizing information in order to make a decision.
The word date back from early 90's, when during a conversation between two person one needed to describe something but the word "pensive" was not suited to the sentence and thinking was too immediate.
"You seemed very thinkative today Jim! Is there anything wrong?"