off-limits (adj.)youdaoicibaDictYouDict[off-limits 词源字典]
"forbidden," by 1881, U.S. military academies jargon, from off (adv.) + limit (n.). Earlier (1857) it was applied to cadets, etc., who were in violation of the limitations on their movement and behavior.[off-limits etymology, off-limits origin, 英语词源]