Realizing full coverage, low-maintenance, and low-cost tactile skin is a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">de facto</i> design dream since the invention of robots. It ensures safety and enables collaborative work protocols for human robot interactions (HRI). The on-robot tactile capability is realized by deploying an array of external sensors or inferring from proprioceptive information that comes with the robot, such as motor torque. However, these methods may be cumbersome, introduce extra management cost, expensive, lack real-world robustness, or require special robot designs. In this letter, we present <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SonicSkin</i> , a low-cost ($2) and easy to deploy system that localizes the on-robot human touch and estimates the touch pressure without actually attaching sensors at potential touch locations. The system requires only a single pair of piezoelectric transducers ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i.e.</i> one transmitter and one receiver) attached on the target robot and turns the robot itself into a versatile sensor. We present a set of novel algorithms to progressively address the unique challenges posed by our system design. We put together an end-to-end <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SonicSkin</i> system on a Jaco robot arm that runs in real-time, and conducted an extensive real-world study including 57019 actual evaluation datapoints under various challenging conditions from 12 human subjects. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SonicSkin</i> achieves less than 2 cm localization error for 96.4% of touches, with more than 96.7% cross-correlation similarity between the predicted touch pressure and the ground truth touch pressure.