Use 'lie back' to describe someone getting physically comfortable, often before resting.

Physical Relaxation

'Lie back' suggests a leisure action with no intention to sleep.

Pure Leisure

Often a solitary action, 'lie back' implies personal space and time.

In a non-physical sense, 'lie back' involves accepting something without responding actively.

'Lie back' can describe a lack of initiative or unwillingness to take action.

The phrase may indicate acceptance of a situation temporarily, often while awaiting change.