bent back
When the back forms an almost "C" or bow shape caused by leaning FORWARD. Bent back is the opposite of arched back, which is caused by leaning backwards. In this the middle of the spine is further behind/up than the top and bottom of the spine.
Sometimes seen when spooning or a character is otherwise curled around something, eg. Danbooru post #7203233.
Not to be confused with backbend.
Examples
For the less extreme version use slouching.