What connotation does, "to have something on someone" have?
Solution 1:
to have something on someone YourDictionary.com
you know some fact or secret, or that you have some specific skill or position, that gives you power over another person.
An example of having something on someone is knowing your friend is has 'crush' on X, and you threaten to tell X of this if your friend does/ does not do Y.