“Frolic” (“make merry, have fun, romp playfully”) comes from the Middle Dutch vrolyc “happy,” which is a combination of vro- “merry, glad” + lyc “like” (as in “similar”). The PIE root (*preu- “to hop”), also the root of “frog,” gives “frolic” the sense of “jumping for joy.”
Sometimes appearing with the spelling “frolick,” especially in earlier texts, the word first romped into English in the 1530s as an adjective meaning “joyous, merry, full of mirth.” That spelling remains in the forms “frolicked” and “frolicking.” Nowadays, the rare adjective form is “frolicsome.” It was first used as a verb in the 1580s, and can also be used as a noun (c. 1610).
Vro-, the first part of the Middle Dutch vrolyc, is a cognate with the Old Norse frar, meaning swift and the Middle English word frow, meaning “hasty,” both of which are also from the PIE preu-. The German word fröhlich, “happy,” shares a similar etymology.
The word “rollick,” meaning “be jovial in behavior,” arose in 1811 as a combination of “roll” and “frolic.” It was first used in the form “rollicking,” but it appeared as “rollick” in an 1826 entry in The Journal of Sir Walter Scott.
Useless Etymology 