Here's what I wrote in my reply:
I just typed a huge long reply, but it disappeared before I hit the reply button!
Basically, to shorten the long missive I wrote before, the word sum for "facetious" is:
Facet + i + ous --> facetious
"Facet" is a bound base (meaning it's not a word on its own), and it comes from a Latin root Facetus, meaning witty/a joke. In Facetus, the <us> is a Latin suffix, so the base is Facet.
We know <ous> is a common suffix, found in words like dangerous and famous.
And the <i> ? What's that?
Well, it's a "connecting vowel letter". My previous response had a long explanation of it, but I think it's best if I refer you to the source of the information - on the realspelling website there is a great 'Gallery' with many slideshows showing various spelling conventions and concepts. The slideshow about 'Connecting Vowel Letters' can be found here: http://spelling.phanfare.com/5232742
I'm wondering if you can see the connection to that other 'confusing' suffix, *tion?
It's confusing because people can't remember when to use *tion, or *sion, or even *cion.
And it's no wonder they're confused!
There is NO such suffix!
The <t> and <s> always, always come from the BASE!
For example:
act + ion --> action
protect + ion --> protection
process + ion --> procession
It's all about that BASE!