the.com/open and shut case

means A situation or legal matter so clear-cut that the outcome is obvious from the start, with no real room for doubt or debate.

from This phrase comes from the physical simplicity of opening and shutting something — a door, a book, a case file — with no fuss or complication involved. It emerged in legal slang around the 19th century, describing trials where the evidence was so decisive that the court could essentially 'open' the case and 'shut' it again almost immediately. From there it spread into everyday speech for any matter — a mystery, a decision, an argument — that resolves itself with minimal effort or ambiguity.