C16: via French carrousser from German (trinken) gar aus (to drink) right out
Word Origin and History for carouse
v.
1550s, from Middle French carousser "drink, quaff, swill," from German gar aus "quite out," from gar austrinken; trink garaus "to drink up entirely." Frequently also as an adverb in early English usage (to drink carouse).