reason
"The farmer checked on why, a young heifer that had just given birth for the first time."
A young heifer.
In plain English: A why is a reason for something happening.
"The detective asked her why, but she didn't have an answer."
For what cause, reason, or purpose (interrogative adverb).
"Why did you decide to cancel our plans for tonight?"
Introducing a complete question.
"Why are you running late?"
In plain English: Why is used to ask for a reason or explanation about something.
"I cannot tell you why she left without asking her directly."
Usage: Use "why" to ask about the reason or cause behind an action by placing it at the start of a direct question like "Why did you leave?" It is distinct from "where," which asks for location, and should not be confused with "whence," an archaic term no longer used in everyday speech.
An exclamation used to express indignation, mild surprise, or impatience.
"Why did you leave my keys on the table again?"
The word "why" traveled into English from Old English, where it originally meant "by what" or "how." Its roots go back even further to Proto-Indo-European, evolving from a form related to the words for "who" and "what."