Presumably means “it's reasonable to assume” or “it seems likely based on what we know.” When you say something is presumably true, you're making an educated guess based on evidence or logic, even though you don't have absolute proof.

If your friend isn't at school and you heard she had a bad cough yesterday, you might say she's presumably home sick. You haven't confirmed it, but it's the most logical explanation. When a detective examines clues at a crime scene and says the thief presumably entered through the window, she's drawing a conclusion from the evidence, though she didn't actually witness it.

The word adds a note of careful uncertainty. It's more thoughtful than just guessing wildly. You're saying: “Based on what I know, this seems to be the case, but I could be wrong.” A scientist might write that an ancient tool was presumably used for hunting. A teacher might say that students who didn't turn in homework presumably forgot about the assignment.

Using presumably shows intellectual honesty. You're making your best judgment while admitting you don't have complete certainty. It's the difference between stating something as absolute fact and acknowledging the limits of what you actually know.