easy as pie

Etymology:

• A pie is not easy to make, so the comparison is not related to making a pie. It is in fact related to having or eating a pie. The phrase came to existence in the 1900‘s in the United States of America. A pie was use to represent something that was pleasant and exchanged at joyous occasions. In 1855, the phrase, in a slight variation was published in the book called ‘Which? Right or Left?’ Here it was used as ‘nice as a pie’. Prior to this Mark Twain used the phrase ‘as polite as a pie’. It is also believed that this idiom was originally derived from the aboriginal New Zealand phrase “pie at” or “pie on” from the Maori word “pai” which means “good” in 1920s. Though much realistic evidence of the idiom are related to the Saturday Evening Post on 22 February 1913, and by Zane Grey in “The Young Forester” in 1910.
• Another possible theory of the origin of the phrase may be related to the expression “like eating pie,” originally printed in 1886 in “Sporting Life”. Source: theidioms.com