(1):

(a.) Incapable of being perceived by the senses; imperceptible. Hence: Progressing by imperceptible degrees; slow; gradual; as, insensible motion.

(2):

(a.) Destitute of the power of feeling or perceiving; wanting bodily sensibility.

(3):

(a.) Not susceptible of emotion or passion; void of feeling; apathetic; unconcerned; indifferent; as, insensible to danger, fear, love, etc.; - often used with of or to.

(4):

(a.) Not sensible or reasonable; meaningless.