How can they expect to communicate effectively with others if they don't know the standard, common, agreed upon definitions/meanings of terms (with all their variations depending on context ) or if they have their own, personal, different definitions/meanings according to their own views and reality?
One can always of course describe one's own definition/meaning of a term --nothing bad about it-- but at least they should make that clear if that definition/meaning departs from the standard, common, agreed upon definition and meaning. Isn't that right? — Alkis Piskas
I am going to answer this question, or pair of related questions, without recourse to a dictionary, by way, in part, of illustrating that communication is possible without recourse to dictionaries. Indeed it must necessarily be the case that the meanings of words were already established and communicated before the first dictionary was created to record them. My answer will also give an answer in passing to your first question.
Fortunately, language has a large measure of redundancy, such that even if *unreadable splodge*, the meaning can very often be discerned quite easily. This means that an unfamiliar word can be guessed at from its context, and by triangulation with another occurrence in another context, a fairly clear idea can be obtained as to the meaning. And this method of providing uses in context is very often part of a good dictionary entry.
Because "meaning is use". Now a dictionary
uses words, to define each word, and one has to understand the words in the definition to grasp the meaning of the word in question. So there is always more work to be done if one is sufficiently insistent, until as must happen, one finds that the dictionary itself loops around and the definitions become circular. At which point one has have recourse to familiarity with the language as used informally anyway. One does not learn to talk in the first place at least, from a dictionary, but from social interaction.
None of this is to denigrate dictionaries, they are fun and useful in equal measure. And if you want to reference one now and then, it can sometimes circumvent a deal of wrangling in philosophical discussion. But in my experience, it is the little common words like "I" and "is" and "thing", and "meaning" that no one defines or bothers to look up, that cause all the big philosophical problems, and here is exactly where a dictionary definition is no help at all.
https://edition.cnn.com/2017/08/17/politics/tbt-clinton-grand-jury-testimony/index.html
Mind you, if you want to really disappear down a rabbit hole of definitional circularity, I can recommend the classic text,
The Meaning of Meaning. by Ogden and Richards, They find about 16 different uses of the term "meaning" by reputable philosophers, and discount a few more as illegitimate - as in 'the meaning of life', for example. I mean, why be satisfied with dictionary.com, when you can have a book length discussion of a word?