1 : something (as a surviving custom) that remains from what is past
2 a : disagreeable physical effects following heavy consumption of alcohol or the use of drugs b : a letdown following great excitement or excess

http://www.merriam-webster.com/dictionary/hangover

Medical Definition From MedTerms.com

Hangover: A common nonmedical term for the disagreeable physical effects following excessive consumption of alcohol (or the use of other drugs). Veisalgia is the little-used medical name for the condition. It is also called a Spanish headache. http://www.medterms.com/script/main/art.asp?articlekey=26220

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