The word "serendipitous" derives from "serendipity," which was coined by Horace Walpole in 1754. He formed it based on the Persian fairy tale, The Three Princes of Serendip, where the heroes make unexpected discoveries. "Serendip" itself comes from the Arabic name for Sri Lanka, Sarandib, ultimately from Sanskrit.  Walpole created the word "serendipity" to describe the gift of making fortunate discoveries by accident, inspired by the fairy tale.  Therefore, the adjective "serendipitous" signifies something found or happening by chance, often with a positive or fortunate outcome. 

The noun "serendipity," which was coined by Horace Walpole in 1754. Walpole created "serendipity" based on the Persian fairy tale "The Three Princes of Serendip".  According to the Oxford English Dictionary, the word serendipitous was first used in English in the 1950.  Specifically, the OED's earliest evidence for the adjective "serendipitous" is from 1958, in the Times Literary Supplement. The word is formed by adding the suffix "-ous" to the noun "serendipity". 

Examples of "serendipitous" used in a sentence: 
*"Early examples of organic reactions and applications were often serendipitous."
*"To some degree, the delays in the production proved serendipitous." 
*"In a serendipitous coincidence, the Arts Council's new plan included a focus on criticism as an art form." 
*"The comet was thus a serendipitous discovery, but one that quickly overshadowed the results from their main observing program." 
*"I bit my lip as a whisper started in my head, suggesting that such a serendipitous phone call could only mean that things were looking up." 
*"And in a serendipitous twist of fate, he is now in a position to offer my band discounted studio time at a local recording studio."

Examples of "serendipity" in a sentence:  
"You might say this is serendipity, but you really have to make these things happen." 
"Evolution seems to proceed not by design but by chance and serendipity." 
"Success often depends on serendipity and clues turned up by other investigations." 
"His work still evokes the dreaminess and serendipity of a stroll through the streets.:
"Through a bit of serendipity, I was asked to give my take."