Direct Opposites:
* Swallow: This is the most straightforward opposite, as it refers to keeping something in the mouth instead of expelling it.
* Retain: This implies holding onto something, the opposite of expelling it.
Less Direct, but Meaningful:
* Inhale: While this doesn't directly oppose expectoration, it is the opposite action in terms of airflow.
* Suppress: This implies preventing something from coming out, which could be relevant in the context of expectoration.
More Specific, Depending on Context:
* Absorb: This could be used if the context is about expectorating mucus or saliva, as the opposite would be absorbing it.
* Keep in: This is a general phrase that could work if you're talking about something being expelled from the mouth.
It's important to consider the specific context of your sentence to choose the most appropriate antonym.