Green’s Dictionary of Slang

(US) an unstable, eccentric person, thus as adj.

1982		Hayano Poker Faces 47: Extremely loose, action-provoking players [are] called beserkos or desperados.
1990		R. Kane Veteran’s Day 30: Here comes this kid. Beserko. Hysterical [HDAS].
2002		www.wellspouse.com 🌐 I know I get the same reaction and since it takes so long to tell them how it’s going and not sound like a beserko [...].
2013		T. Piccirilli Last Whisper in the Dark 206: ‘Brother went a little berserko and paid for it in the chair’.