-su Godan/u-verb

1. to return (something); to restore; to put back (esp. 返す)
2. to turn over; to turn upside down; to overturn
3. to pay back; to retaliate; to reciprocate (esp. 返す)
4. to respond (with); to retort; to reply; to say back

aux-v

5. to do ... back (e.g. speak back, throw back) (after the -masu stem of a verb)
6. to do again; to do repeatedly (after the -masu stem of a verb)