放肆 (fàngsì) describes brazenly presumptuous behavior that oversteps social boundaries, carrying a strong negative tone (often used for scolding). It is more aggressive and disrespectful than 冒昧 (màomèi), which is milder and used in polite apologies for taking a liberty. It is far stronger than 随便 (suíbiàn), which simply means 'casual' or 'informal' without negative judgment.