Geez, which online casino did you use?  Please tell me lol.

Quite possibly 100% of your winnings.  On-line gambling is illegal in the US and any winnings are potentially subject to seizure when you attempt to repatriate the funds.  You may find it difficult or impossible to get the money transferred to your US bank.  Most of the foreign on-line casino owners are running scared as several have been arrested when arriving on US soil since they transferred gambling winnings to US citizens or residents in the US.  As a result, most of them will not transfer funds to a US financial institution.But let's assume that you somehow dodge the bullet on that.  Those winnings are taxed as ordinary income.  MA will snag 5.3% and your Federal bite will depend upon your marginal rate.