Abarim Publications' online Biblical Dictionary
According to Lewis and Short's A Latin Dictionary, the Latin adjective clemens (clementis) originally denoted the quiet, placid, pleasant state of the air, wind or weather.
It was specifically applied to the gentle motion of the sea and rivers, and even in the sense of a "pleasant, calm" (not steep?) ascent.
Our adjective then became applied to describe calm, unexcited or passionless states of mind, and from there a mildness in respect to the faults and failures of others.
The use in English of this adjective and its associated noun clemency denote a mercy shown to a criminal in a legal procedure. This usage appears to be not implied in Latin.