underthink

From under- +‎ think.

underthink (third-person singular simple present underthinks, present participle underthinking, simple past and past participle underthought)

1. To give insufficient thought or consideration to; fail to consider adequately.
   • 2007, Brian Haig, Man in the Middle:

     There was a new one; usually, I underthink these things.

   • 2008, Jeffrey Kluger, Simplexity:

     This gives us plenty of opportunity to overthink — or underthink — the problem, and this is where we really start to bollix things up.

• overthink

• underthinker