I might say to them that verisimilitudinous is not the usual word (despite the OP). It's the more elegant verisimilar, and thus verisimilarly.
This happens all the time! Glad it’s not just me.
Not really. Something is verisimilar when it has the appearance of truth, or is likely to be true, regardless of whether it is in fact true. In Spanish, verosímil ('verisimilar') is rather frequently used in contexts in which one hypothesizes about the trueness of claims or statements, especially when it cannot be verified at that very moment, or ever.
> Something is verisimilar when it has the appearance of truth, or is likely to be true, regardless of whether it is in fact true.
Hmmm ... in English I've never seen the usage "likely to be true". There are many other words for that - probable, likely, etc.