It's never "just."

Among the many words we use without thinking, "just" ranks pretty highly. Unfortunately, it can also rankle.

"So, what will it take to fix the reactor?"
"Oh, we just need a new hydroelectric magnetosphere regulator."

This is a rough paraphrase from the classic game Fallout 2. The quest giver is a kindly ghoul named Harold, who has a tree growing out of his head. What Harold fails to mention is that you have to do the following to actually get the job done:

  1. Amplify Plutonium-Gamma Shield
  2. Deharmonize Neptunium Impeller
  3. Calibrate Uranium-Rod Driver
  4. Set Voltage on Saturn-Class Capacitor
  5. Test Jupiter Wave Complier
  6. Install hydroelectric magnetosphere regulator to save the plant or turn cooling valve off

In fact, Harold doesn't even know you have to do all of that. He leaves the actual execution to you. (If you're really curious, you can learn all about this quest at the Fallout Wiki.)

In our experience, that's the effect that the word "just" has—it elides over the effort required. Combined with its ubiquity in our daily speech, this is a potent recipe for disappointment. In our experience, this plays out different for developers and customers, so it's worth exploring these separately.

Customers

What they think just means when a developer says "just": "This will be simple and not cost much money."
What the developer probably means: "I think it's going to be easy, but that doesn't mean it won't be complicated."

Developers

What they think it means when a customer says "just": "I know better than you how hard this is."
What the customer probably means: "I'm not sure about the software part, but process-wise, this is simple."

Clearly, this mismatch can cause some issues. Maybe you've hired a developer who ends up being much more expensive than you expected. Or maybe it feels like your client doesn't respect how complicated a project is.

Both cases end up boiling down to a simple problem: saying "just" shortcuts the discussion and consideration that's necessary at the beginning of a task. Instead, it implies a level of certainty that's nearly impossible to have.

In short?

Just don't say "just."