The original lexical meaning of the word just was to mean ‘deserved’; as in:
He got his just reward
… And in other such idioms.
Today, however, it has become a discourse marker. A discourse marker is a word which has a function at a discourse (i.e. conversational) level, not within a sentence or clause.
For example, compare the use of the word like in the following examples
- I like chocolate
- My chocolate is like your chocolate, only yummier
- And I was like, ‘Oh, my God! This chocolate is amazing’
- And I, like, really, enjoyed eating it.
In number 1, we see the word like being used with it’s own lexical meaning, indicating that you find something pleasurable or agreeable.
As used in 2, like indicates that something is similar to something else.
As used in 3, it is an introducer of dialogue (You can read a separate a post on quotative verbs here).
As used in 4, however, it seems to have little function alone; on a conversational level, however, it suggests the speaker’s tone, their meaning. Indeed, they are communicating more than they are saying on a surface level.