ball (n.2)youdaoicibaDictYouDict
"dancing party," 1630s, from French, from Old French baller "to dance," from Late Latin ballare "to dance," from Greek ballizein "to dance, jump about" (see ballistics). Hence, "very enjoyable time," 1945, American English slang, perhaps back to 1930s in black slang.
hop (n.3)youdaoicibaDictYouDict
"a small jump," c. 1500, from hop (v.). Slang sense of "informal dancing party" is from 1731 (defined by Johnson as "a place where meaner people dance"). Meaning "short flight on an aircraft" is from 1909.