The word "rationalize" is formed from the adjective "rational" and the suffix "-ize".   The word "rational" comes to English via the French word "rationaliser", which in turn comes from the Latin word "rationalis", meaning "of or belonging to reason," which itself comes from ratio, meaning "reason" or "calculation". Therefore, to "rationalize" literally means to "make rational" or "to explain something in a rational way",   The word rationalize has evolved to mean the psychological act of giving a plausible but false reason for behavior to conceal the true motive.  It can also mean to make something more efficient or effective, as in "rationalizing operations". 

According to the Oxford English Dictionary (OED), the earliest known use of the verb "rationalize" is from the mid-1600s, specifically 1665, in the writing of N. Hodges.  The most common modern meaning emerged in psychology.  It was first introduced by psychoanalyst Ernest Jones in 1908, and this sense means "to devise superficially plausible explanations or excuses for one's actions, beliefs, or desires, often without being consciously aware that these are not the real motives".

Examples of using "rationalize" in a sentence: 
"Sarah tried to rationalize her grandson's strange behavior by blaming it on the boy's father." 
"He couldn't rationalize buying such an expensive car." 
"John didn't bother trying to rationalize what the sound could have been."  
"Jessica couldn't rationalize her sudden urge to return to the cottage." 
"So what, then, is the big problem with simply trying to rationalize the system?" 
"You can easily rationalize it by telling yourself it's work-related." 
"Ernest selfishly tried to rationalize his decision by saying it would be better for everyone. " 
"How did the individuals who masterminded and ran the camp system rationalize their own behavior?" 
"Hard to believe that these people can justify and rationalize such sickening behavior!" 

If I didn't have a reason for commenting on word of the day I could easily rationalize one.

Just the facts:

RATIONALIZE Definition & Meaning - Merriam-Webster

RATIONALIZE Synonyms: 13 Similar Words | Merriam-Webster Thesaurus