drunken bash; rave party
A wild, noisy party with heavy drinking; often implies a chaotic, uninhibited atmosphere.
Last night's drunken bash went on until morning.
He made a huge racket at the rave party.
乱痴気 (らんちき) is a compound of 乱 (disorder), 痴 (foolishness), and 気 (spirit), suggesting a wild, foolish mood. Combined with パーティー (party), it describes a drunken, raucous gathering. The exact origin is uncertain, but it likely emerged as a slang term.