If you're thinking of the basic definition of "someone you know casually":
* Stranger: This is the most direct opposite, meaning someone you don't know at all.
* Unknown: Similar to "stranger," this emphasizes the lack of knowledge about the person.
If you're thinking of the sense of "someone you have a passing familiarity with":
* Intimate: This emphasizes a deep, close relationship, the opposite of a superficial connection.
* Friend: While not always the exact opposite, "friend" implies a stronger bond and deeper understanding than an acquaintance.
If you're thinking of the sense of "someone you've met but don't know well":
* Close friend: This highlights the difference in level of closeness and familiarity.
* Confidante: This implies a level of trust and intimacy that an acquaintance wouldn't have.
Ultimately, the best opposite for "acquaintance" depends on the specific context you're using it in.