辜 is a phono-semantic character. In the seal script, 辜 
= 
(古)+ 
(辛).
辛, a square knife used to tattoo a criminal’s face for identification, is the semantic component.
古 means past or ancient and serves as the phonetic component, indicating that the crime is convicted in accordance with past cases.
The original meaning of 辜 is death penalty.
Its extended meanings are:
A. crime, guilt, for example, 死有余辜 even death would be too good for sb., 无辜 be innocent
B. let down, fail to live up to 辜负, 你辜负了他的期望。You‘ve let him down.