

The general meaning is the appearance of truth or validity.
But I usually use it to describe something that is “believable” even if the underlying premise is not. So a fantasy story that pays close attention to detail and is highly consistent might be described as having versimilitude. On the other hand, a story where the characters make out-of-character choices might be lacking versimilitude, even if there are no overtly “fictional” elements to the story.
That’s usually how I’ve heard it used, not sure if it’s the “main” usage though.
It’s like the wholesome counterpart to “boob.” Both kinda sound like what they describe, but “bosom” feels classy.