The word — meaning gentleness or mildness — comes from Latin mansuetus, tamed or made gentle. It contains manus, hand (from which English got manicure, manufacture, command, manual and other words), and suetus, accustomed (which is from the verb suescere, the source of a few words, of which the best known is desuetude). The idea behind mansuetus is that if an animal has become accustomed to the hand, it has been tamed.
An example of the English word appears in The Letter of Marque by Patrick O’Brian, in which Dr Stephen Maturin is extolling the virtues of laudanum: “Presently, with the blessing, you will see Padeen’s face return to its usual benevolent mansuetude; and a few minutes later you will see him glide insensibly to the verge of an opiate coma.” This is a more recent example:
While her voice may have an air of mansuetude, she proved that she could easily cut above the din of the boys in her band.
Boston Globe, 24 Jun. 2004.
The word is not entirely obsolete, though it is rare to the point of being marked as archaic in most dictionaries and is definitely literary even when it does appear. To use it may be to gratify the ego of the writer rather than communicate with the reader.