Gleeful means full of delight, often at another's cost. A gleeful grin, gleefully reported, gleeful malice.
The at another's cost part is not required but is usually there, which is what gives the word its edge — gleeful is delight somebody is enjoying rather more than they should. Note glee, which in music means a part song for several voices, an entirely separate sense.
1adjectivegleefulFull of delight, often at another's cost.
“My brother was gleeful when I got told off for the mess he had made.”
gleeful is a quality of delight.
Glee plus -ful, from Old English gleo, entertainment or music. The musical glee comes from the same word.
Proto-Indo-European
*gʰlew- — to joke, make fun, enjoy
Proto-Germanic
*glīwą — joy, mirth
Proto-West Germanic
*glīw
Old English
glēo
Middle English
gle
English
glee
English, today
gleeful
Built from
From glee + ful. glee comes from Middle English gle, from Old English glēo, glīġ, glēow, glīw (“glee, pleasure, mirth, play, sport; music; mockery”), from Proto-West Germanic glīw, from Proto-Germanic glīwą (“joy, mirth”), from Proto-Indo-European gʰlew- (“to joke, make fun, enjoy”). Cognate with Scots gle, glie, glew (“game, play, sport, mirth, joy, rejoicing, entertainment, melody, music”), Icelandic glý (“joy, glee, gladness”), Ancient Greek χλεύη (“joke, jest, scorn”). A poetic word in Middle English, the word was obsolete by 1500, but revived late 18c.
Etymology from Wiktionary, CC BY-SA