Winning a round (generally) obligates you to deal the next round. Part of doing that is finding a word to deal. Linked on the menu are several dictionaries that may help you in that quest.
There are only two remaining “unabridged” dictionaries of English that have editorial teams keeping them up to date. They are the Merriam-Webster Unabridged and the Oxford English Dictionary, 3rd edition. Both are (for obvious reasons) behind paywalls. The links here point only to dictionaries you can consult for free. Other than Wiktionary, they are at best sporadically updated. There are also links to older dictionaries that in their day commanded respect comparable to the respect today accorded to Merriam-Webster and the OED. Dictionaries do not lose stature or authority with the passage of time: only their relevance to contemporary usage. Players regularly choose words and definitions from dictionaries that originally appeared a century or two ago.
The earliest advice on choosing a word was by "Rules Mother" and charter player Theresa Carey, written in 1989, in response to a single controversial choice of word. You may not find it very helpful; it is primarily here as archival material.
There have been many debates on the subject since then. You will find an up-to-date summary in the comments to Rule 1(a) of The “Real'”Rules. You can also read there why opinions are divided over other online dictionaries, such as Urban Dictionary and The Free Dictionary.
Be sure to do a Similarity Check of any candidate word you pick.