If you mean "to eject saliva from the mouth," then the opposite could be:
* Swallow: To move food or liquid from the mouth to the stomach.
* Retain: To keep saliva in the mouth.
If you mean "to eject something forcefully from the mouth," then the opposite could be:
* Suck: To draw something into the mouth.
* Hold: To keep something in the mouth.
If you mean "to speak rudely or contemptuously," then the opposite could be:
* Praise: To express admiration or approval.
* Respect: To show honor or esteem.
If you mean "a small amount of something, especially saliva," then the opposite could be:
* Gush: To flow out in a large amount.
* Flood: To cover completely with water.
Let me know which meaning of "spit" you had in mind, and I can give you a more specific opposite!