While playing Quiddler tonight, I played both words "SIC" and "QUO". Both got challenged and both were in the dictionary that we agreed upon (dictionary.com).
But the question of whether I can use "QUO" because the dictionary defined it as "Verb - Archaic: Quoth."
I said it's allowed because it's in the dictionary and used in common vernacular.
My opponent said I couldn't use archaic English words because they are no longer part of the English language.
Is this true for Quiddler? Scrabble?
The word legality rules of Quiddler appear to be slightly more relaxed than those of Scrabble - according to Wikipedia, capitalized adjectives such as "Iraqi" and "Scottish" are acceptable in Quiddler, where to the best of my knowledge they still aren't in Scrabble.
Archaic words, by contrast, have always been allowed in Scrabble. Again per Wikipedia:
"Variant spellings, slang or offensive terms, archaic or obsolete terms, and specialized jargon words are allowed if they meet all other criteria for acceptability."
So I'd say they were definitely fair game for Quiddler too. Having said all that, both Scrabble and Quiddler are games that lend themselves well to house rules. If you're playing with your grandmother you're probably going to dial back on the offensive terms; likewise some people get so offended by the idea of allowing slang or archaisms that it can become easier just not to use them!
As a side note, the funny thing about "quo" is that I suspect most people try to use it as part of the phrase "status quo" - which would be a foreign phrase and therefore not acceptable! But luckily your dictionary contains a legitimate form of it also.
To add to thesunneversets' answer:
The words allowed in Quiddler are those in a pre-agreed dictionary of the players' choice.
Similarly for Scrabble, the players may agree on a dictionary. However, there are also word lists compiled especially for Scrabble. In (English language) tournaments in most countries, as well as for the World Scrabble Championship, SOWPODS is used. National tournaments in the USA, Canada and Thailand use TWL.
Scrabble has an official dictionary (the Official Scrabble Players Dictionary) to eliminate disputes of this kind. It's available online on Hasbro's website.
I'd recommend sticking to this for any word-based game (such as Quiddler) that does not specify an alternative authoritative dictionary in the rules.
Note: "sic" is in the OSD 6th edition, but "quo" is not.
