Someone asked me privately if “optissimus -a -um” is a real Latin word. I have seen the word in certain reputable (late) sources but it’s very rare, so I changed it to “optima”. [Well, I did write “the best” and not “the very best”.]. So is it better to treat it as a dopey mistake, or does it properly mean “the very best”? Personally, I think it does mean that (“the very best”),—as a superlative of a superlative!
Quidam me clàm rogavit necne genuinum verbum sit “optissimus -a -um”. Id genuinum esse credo quià in quibusdam fontibus bonis et seris invenitur etsi rarissimé. Ideò, estne verò vitium stultum, an anglicè “the very best” sibi vult? Ego puto id sic sibi velle,—superlativum superlativi.
Where have you seen it used?
I read it elsewhere but, when I searched just now, I found it’s classical and Ciceronian.
Alibi id legi sed, cum modò quaesivi, quòd id classicum et ciceronianum est inveni.
Well, if Cicero used, then by definition it is correct, since he is the standard. Breathe easy, amice. 
Or maybe not! Because other sources say “optatissimum” there (in capite uno, Reditum in Senatu).
but it’s in Hieronymi Fracastorii Veronensis, de Sympathia, et Antipathia rerum, Venetiis
MDXLVI (1546) Caput 18 “qui et ad laeta optissimi sunt, et tristibus maxime possunt resistere” (http://ourworld.compuserve.com/homepages/gto/fracastor/ch18-46.html)
Optissimus -a -um also appears as an online spelling mistake (Optissima Pax) for Pope Pius XII’s encyclical Optatissima Pax (1947).
So are its rare occurrences (I know I’ve read it elsewhere, too) shortenings of “optatissimum” (as acceptable alternative spellings) or spelling mistakes or is it a proper word in its own right? Now I’m more inclined to believe that “optissimus” is just a dopey mistake (since Cicero’s sense is more likely “optatissimum” and OED & L&S give it so with Cicero).
Estne synonymum pro “optatissimus” adjectivo an orthographiae vitium an in ipso proprium verbum? Nunc eó magìs adducor ut credam “optissimus” modò vitium ineptum esse.
You’ve already researched this far better than I could hope to, but I concur that “optimus” is correct and “optissimus” is a mistaken form. This sort of error seems common because it’s based on false analogy with the superlative form of most adjectives. This situation is probably only acerbated by the “-issimo” parallel in modern romance languages. I recall there being a similar discussion on here a while back regarding “multissimus” instead of “plurimus.”
With hands up, I confess it’s a fair cop, thesaurus.
Manibus elevatis, thesaure, me aequè prehendi fateor.