What preposition should I use in the expression "put ___ the backseat"?
The sentence goes like this:
I have a few items on my plans, item A is the least important one, so I will put it ___ the backseat.
Should I use "at" or "onto" here?
I think the idiom you are looking for is to take a backseat, which means to be in a position of less importance or a position of not being in control. If you’re in the driver’s seat, you’re in control of the car; if you’re in the back seat, you’re not in control.
Here are some examples of this idiom from the Corpus of Contemporary American English (COCA):
There aren’t any idiomatic uses of
If you are speaking literally about placing objects or people into the back seat of a vehicle, you can use in, on, into, or onto, depending on the circumstance. You can only use into and onto for motions, while in and on can be used for either state or motion. Generally, you use in and into for people, and on and onto for objects, although sometimes people are on a backseat, though typically only if they are not sitting normally. Also, both back seat and backseat appear to be in equally common use. Examples from COCA:
Well you could save yourself some trouble by saying you are going to postpone it.
I'm not familiar with this idiom about back seats. There is an idiom "to put on the back burner" which also means to postpone or delay.
I think the proper use is neither of those choices, but, rather, either "in the back seat" or "on the back seat".
I'd use "on the back burner" as @delete suggests.
Should you use "at" or "onto": There is no fix rule per se, and mostly it's a matter of context, we tend to use one preposition over another. There are many exceptions and overlapping situations.
And if you wish to use a proper idiom, then,as explained,use:
to take a backseat
to put it on the back burner.
Alternatively, use COCA.
"Put it at the backseat," sounds intuitively odd. But there seems to be a rather poetic use of "put" and "at" when one puts something at the presence or body of another person. Here is an example from The Acts of the Apostles, Chapter 4 (New International Version, later 20th Century and linguistically updated):
Actually, I found this page because I searched "put it at" after writing this.