What is the meaning of "Spice up"?

Why the meaning of "Spice up" is not in any dictionary? what is the meaning of this phrasal verb? thanks.

To "spice something up" is to make it more interesting or exciting.

Math can be boring, so I like to spice up my lectures by throwing in a few jokes.

This web page seems too simple -- let's spice it up with a little Javascript.

Thank you so much Jacob.