the product of slope of x-axis and y-axis is not -1 . how it is possible ?

we know that generally the slope means the tan of the angle made by the line with positive x axis for x axis itself the angle is 0 degress => tan0=0 for y axis the angle is 90 degress => tan 90=infinity though product of slopes isn't equal to -1 slope of x axis = -1/slope of y axis (0=-1/infini