The pleasure derived from another person's misfortune. Imported into English from German, like blitz, flak, kitsch, weltschmerz, angst, realpolitik and hamburger.
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“The earliest appearance of the word is found in philologist Richard Chenevix Trench’s 1852 meditation on language, Study of Words. Trench’s citation is mostly a lament. Words such as ‘schadenfreude,’ he remarks, reflect a degraded moral interiority: ‘… what a fearful thing is it that any language should have a word expressive of the pleasure which men feel at the calamities of others; for the existence of the word bears testimony to the thing.’”
(Jane Hu: A Joyful & Malicious History Of ‘Schadenfreude’, The Awl, 20 October 2011)
Now this is compelling: the usage of 'schadenfreude' has been surging in the English literature.
The Google Ngram Viewer is an online phrase-usage graphing tool. It charts the annual count of selected words and phrases, as found in more than 5.2 million books digitized by Google up to 2012. This is how the frequency of the word "schadenfreude" has changed over time in the corpus of the English literature (British and American):