सुहृद्-दुर्हृदौ मित्र-अमित्रयोः

The word सुहृत् is derived from सु + हृदय, the अय of हृदय being elided in the Bahuvrihi. It means one whose heart is favourably disposed towards another. So also दुर्हृत् is derived from दुर् + हृदय, he whose heart is evil.

Why do we say \when meaning a friend and a foe respectively\? Observe सुहृदयः कारुणिकः 'the kind-hearted, merciful'. दुर्हृदयश्चोरः \the evil hearted thief\.,