Originally from MMORPG gaming jargon — 'Player Kill' / 'Player vs Player.' Imported into Chinese in the 2000s, it then leaked into general usage and became massive in livestream culture, where 主播 PK is a built-in feature: two streamers' rooms get linked and viewers gift to push their side ahead.
register
Letter-name pronounced (P-K). Used as verb (PK 一下 'face off briefly'), noun (一场 PK 'a PK match'), and adjective-ish (PK 模式 'PK mode'). Casual register; even appears in news headlines now.
