sit back

[Kit]

be inactive or indifferent while something is happening

[Jacobanade]

settle into a comfortable sitting position

[SergioAnigo]

To relax, to not exert oneself.

[Thu Thi Le]

To sit back generally refers to the act of reclining, relaxing, or resting while refraining from getting involved in something. It often implies a state of inactivity, least involvement, or passive observation. The term can be used literally, such as physically sitting back in a chair, or figuratively, such as not participating or interfering in a situation or not taking initiative.

[juan98]

To recline while still in a seated position, with ones back on the frame of the seat.