16th-century English revelers toasting each other’s health sometimes drank a brimming mug of spirits straight to the bottom — drinking ‘all-out,’ they called it.
German tipplers did the same and used the German expression for ‘all out’ — ‘gar aus.’
The French adopted the German term as ‘carous,’ using the adverb in their expression ‘boire (to drink) carous,’ and that phrase, with its idiomatic sense of ‘to empty the cup,’ led to ‘carrousse,’ a French noun meaning ‘a large draft of liquor.’
And that’s where English speakers picked up ‘carouse’ in the mid-1500s, first as a noun (which later took on the sense of a general ‘drinking bout’), and then as a verb meaning ‘to drink freely.’