I googled it and something came up from merriam-webster online dictionary.
I don't know if this website is a credible source or not but I just wanna know from native speakers out there. Is it really?
Wow! My immediate reaction when I saw your question was "No!" However, I've also now looked it up in every online dictionary I use and it appears in all of them.
In my opinion, one of the most horrible-sounding words I've ever heard!!!
Agree on all counts. It seems it's an AmEng word and almost all references on the first page of google are to a sporting context (eg 'the winningest baseball team').
Amusingly, when you select 'Pages from the UK', the number of entries drops from over 500,000 to under 10,000, the first of which is headed 'Did you know that "winningest" is a word?':lol:
"Winningest" does sound awkward when spoken, but then again I always stumbled over "Isthmus of Panama", too. :lol: