Plagiarism in the usual, legal sense has to do with works published for profit or acclaim. It may be scientific research, books, magazine articles, plays, music and lyrics for sale. It is passing off an idea without acknowledgement that you are not the original source. It is falsely claiming to be the creator of that idea.
On a fundamental level however we are all guilty of plagiarism. If you are able to read this passage in English you are also able to think in English even though you yourself did not invent the English language. In thinking about how you think, you actually think in your “mother tongue” or the words and concepts first learned as a child from your very own mother; and may well be fraught with mythology, superstition and error. The words and ideas currently running through your head are hers and not your own; thus you are a plagiarist.
Of course most of the accumulation of words, ideas and values you carry around in your head have a source other than your mother. So, trace them back and consider the source.
There is reality and then there are the labels we hang on reality. We hang labels on things (nouns) and actions (verbs) and descriptors (adjectives and adverbs) and all manner of abstract concepts and toss them about too often unexamined. The labels, even of old, are, more often than not, both altogether wrong and terribly misleading.
From time to time your ideas and beliefs are challenged by someone and one might well be asked directly: “Wherever did you get that idea?”
Obviously, since we are all at bottom plagiarists, it is, in fact, a legitimate question. Every word, every idea coursing through your head has a source and you are not that source. So, Wherever did you get that idea?