This is a pun on the phrase 'sign of the times' which means something judged to exemplify or indicate the nature or quality of a particular period, typically something unwelcome or unpleasant., but 'sine' is used in it's place because it sounds similar and it means the trigonometric function that is equal to the ratio of the side opposite a given angle (in a right triangle) to the hypotenuse.
The mathematics professor , lamenting his students ' lackadaisical approach to trigonometry , sighed , ' ' It ' s a sine of the times . ' '