There are many idioms and phrases for 'on the back burner'. Some idioms and phrases for 'on the back burner' include 'pushing aside' and 'procrastinating'.
"Idioms for Being Afraid", "Idioms for Being Angry", "Idioms for Being Happy" are some examples of idioms found in the book "Found" by Margaret Peterson Haddix.
Here is a link to some idioms. You can pick the "non-common" ones for yourself -- some people have heard certain idioms a lot and others have not heard the same ones, so you'll need to decide that part.
Some websites that have idioms written in them include The Free Dictionary, Cambridge Dictionary, and Phrases.com. These sites provide definitions and examples of idioms to help users understand their meanings and usage in context.
savvy, brainy
The best place is to go on your search engine and type in "list idioms" You can also look at this question for some examples.
Some examples of idioms in "Divergent" include "jumping off the deep end," "barking up the wrong tree," and "walking on eggshells." These idioms are used to convey deeper meanings or emotions within the story.
Nostalogic
cry unclemonkey's uncle
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confererats