In some cases, "plain" can be an antonym for "ordinary".
* "Plain" can mean simple, unadorned, and straightforward. This can be seen as the opposite of "ordinary" which implies something unremarkable or commonplace.
* Example: "He wore a plain white shirt, not wanting to stand out." (This suggests the shirt was not extravagant, making it the opposite of an "ordinary" shirt.)
However, "plain" can also be synonymous with "ordinary" in some contexts.
* "Plain" can mean common, typical, or average. This meaning is very similar to "ordinary".
* Example: "She ordered the plain burger, not wanting anything fancy." (Here, "plain" suggests the burger was unremarkable, just like an "ordinary" burger.)
So, it depends on the situation. You need to look at the specific context to determine whether "plain" is an antonym or synonym for "ordinary".