不买账

Literally 'not pay the bill' — 买账 (mǎizhàng) means to honor / accept someone's authority or claim. 不买账 = refuse to accept it, no matter what's offered. Maps almost exactly onto English 'not buying it'.

usage

Used both for personal disagreements (refusing to back down before someone with status) and market reactions (consumers rejecting a product). Often paired with 对 X 不买账 ('don't buy [X's] line'). 账 is sometimes written 帐 — older texts, traditional contexts.