How to understand the word "back" in this sentence?

Question

Merry Christmas！And I've got a question to ask on this wonderful holiday.

It's an example sentence under the entry "sink" in New Oxford Dictionary:

(of a person) lower oneself or drop gently: She sank back onto her pillow.

I want to confirm if the word "back" in the sentence is used as an adverb, which means returning to her early lying position. Is it possible that it is a noun which means "her back"? Then the sentence would mean she lay in her bed with her face up.

I would prefer the former explanation, but still can't be very sure to expell the later.

Answer 1

My first answer here so here goes. I'm sure someone will explain exactly why it's the first explanation but I'll give it a shot.

'Sank back' does, as you said, mean return to a previous position. If 'back' was a noun, it would need to be 'She sank her back onto her pillow'.

Answer 2

Actually, there's another possible explanation that I believe is the most likely: this phrasing is used mostly when going from an upright position (generally sitting up straight) to a partially or fully reclined position (laying on your back), which is a backward motion. Since you are both sinking and moving backward, you are "sinking back".

For a difference, if you simply "sink into your chair", you can go from standing upright or sitting up straight into a defensive, hunched-forward posture, with your head down and your elbows on your knees; but if you "sink back into your chair", you are putting your entire back against the chair's back (and, if the chair flexes, you are most likely reclining the chair's back as well).

It is true that "sinking back into your chair" (or pillow, or whatnot) could be used for "returning to your previous position"; hopefully context would make it clear which meaning is intended.