Template:Random Latin fact/nobiscum
The Roman statesman Cicero had a folk etymological explanation for why we say "nobiscum" and not "cum nobis."
Why don't we say cum nōbīs, but rather nōbīscum? Because: If we say it the other way, the letters would run together in a rather obscene way.—Marcus Tullius Cicerō, Orātor ad Brūtum, § 154
The joke is: cum nōbīs can easily be misunderstood as cunnō bīs which has a lewd meaning, that we are not going to explain here.