He seemed tired, and the Rat let him rest unquestioned, understanding something of what was in his thoughts; knowing, too, the value all animals attach at times to mere silent companionship, when the weary muscles slacken and the mind marks time.
to gradually decrease in intensity or tautness; to become slack
Hyphenation
to gra·du·al·ly de·crease in in·ten·si·ty or tautness; to be·come slack