How to automatically populate a field with a value part of another field I will try to be as specific as possible.  

I have a required text plain field in a content type, say 'field_A'.  This will have values of minimum 2 words, maximum 5 words. Words will be separated by spaces.  I can be certain of this. 

What I want, is to have another field, say 'field_B' in which I want to keep the last word of field_A.  I would like this to happen automatically (upon creation of a node of this type). 

I have no idea if this is possible ... 

So, say I 'm adding new content of this type, and use value "lorem ipsum whatever" for field_A, upon submitting I would like the word 'whatever' to be inserted as a value in field_B.

I 'd be grateful of any help ..  I couldn't find relevant info on this, it's kinda hard to describe in a few words. 

Working on D8.
You probably need a custom module for this. 

Add  MODULENAME directory inside your modules directory.  Inside the directroy create file MODULENAME.info.yml with contents:

name: Last word copy
description: 'gets last word from field_a and saves it to field_b'
core: 8.x
type: module
package: COMPANYNAME
dependencies:
  - node


This module implements hook_form_alter that will make field_b readonly / disabled and hook_entity_presave which will get the last word added to field_b upon save or update of a node of CONTENTTYPE

Create also MODULENAME.module file with contents:

&lt;?php
function MODULENAME_form_alter(&amp;$form, &amp;$form_state, $form_id) { 
  if(in_array($form_id,['node_CONTENTTYPE_form','node_CONTENTTYPE_edit_form'])) {
    $form['field_b']['#disabled'] = 'disabled';
  }   
} 

function MODULENAME_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {  
  if($entity-&gt;bundle() == "CONTENTTYPE"){    
    $pieces = explode(' ', $entity-&gt;field_a-&gt;value);
    $last_word = array_pop($pieces);
    $entity-&gt;field_b-&gt;value = $last_word;
    //save is done automatically
  }  
}


You NEED to replace MODULENAME and CONTENTTYPE in the above code. 

If all is correct go to Extend (admin/modules) and enable the module.