"Starts in on" is a phrase that typically refers to beginning a task or activity with focus and determination. Antonyms of this phrase could include "gives up," "abandons," "quits," "departs," "disengages," or "avoids." These words imply a lack of dedication or commitment to a particular task or activity. On the other hand, synonyms for "starts in on" could include "commences," "begins," "initiates," "launches," "undertakes," or "embarks on." These words convey enthusiasm and determination towards the task at hand. Understanding antonyms and synonyms can help expand one's vocabulary and better articulate thoughts and actions.