Wiktionary
sit back
vb. 1 To recline while still in a seated position, with one's back on the frame of the seat. 2 (context idiomatic English) To relax, to not exert oneself.
WordNet
sit back
v. settle into a comfortable sitting position [syn: take it easy]
be inactive or indifferent while something is happening; "Don't just sit by while your rights are violated!" [syn: sit by]