A hangover — the unpleasant physical effects (headache, nausea, fatigue) experienced the day after heavy drinking.
昨夜は飲みすぎて、今日はひどい二日酔いだ。
I drank too much last night and have a terrible hangover today.
二日酔いには水をたくさん飲んで休むのが一番だ。
For a hangover, drinking lots of water and resting is best.
会議があるから、二日酔いにならないように気をつけてください。
Please be careful not to get a hangover since we have a meeting.