Note: Finding the origins for popular sayings can be very difficult a lot of the time, as it's hard to find that one person or even area where an idiom started. For the most part, what's given are possibilities in how a phrase could have originated, but not necessarily how it actually did. Additionally, early recorded forms of a saying will be given, and these tend to come from old books, newspapers, poems, or plays. However, if phrases are being printed in forms of media, like a newspaper, that's usually a sign to show it's already a well known saying, and is probably much older.
Ultimately, you get an approximate on how long a phrase has been used for, and gain a little understanding on where a phrase's roots are from.
Know Your Phrase has lists of idiom meanings, and more are being added!
knowyourphrase.com - Sayings and Phrases - Meanings and Origins
An activity that requires little effort to finish; a job that's simple; easy.
When a task is easier to complete than previously expected, people will
sometimes use this phrase to express those sentiments.
That's as easy as pie! Wait a second, that's the wrong phrase, but it's meaning is identical to a piece of cake; both expressions convey the idea of simplicity. The question is: Why? What makes cake "easy," anyways? Well, I doubt it's referring to the cooking process involved with baking a cake, because that requires a fair bit of work. You have to crack eggs, mix stuff up in a bowl, set the temperature of the oven, and so on. Alright, it's not that complicated, but it still requires effort, so much so that some people don't even care to bother with it.
Do you know what is easy though? Eating a baked cake! Yeah, that's a pretty easy thing to do because it tastes good. Thus, this phrase may possibly get its "easy to accomplish" meaning from how simple it can be to eat a piece of this delicious desert. However, that is simply a guess; the origins for this phrase are not certain.
The figurative meaning of this phrase goes back to at least the 1930s. The term is used by an American poet named Ogden Nash, who wrote Primrose Path in 1936. There's a quote from it that reads:
"Her picture's in the papers now, And life's a piece of cake."
* This upcoming bicycle race will be a piece of cake for me because I've been going through exhaustive training sessions these past few months to prepare myself for it!
* Blake's roof was leaking, so he climbed to the top of his house to fix the problem thinking it would be a piece of cake, and it was!