The antonyms for the word "un lax" are strict, stringent, tight, and rigid. These terms imply a high level of control and discipline. When we refer to something as strict, we mean that it is closely regulated, and there is no room for leniency. Stringent describes a condition that is extremely strict, with stringent regulations that are strictly enforced. Similarly, tight refers to a situation where there is no room for error, and everything is tightly controlled. Rigid is slightly different, implying a lack of flexibility, with set guidelines or procedures that must be adhered to strictly. These antonyms are good choices when we want to communicate the opposite of an unrelaxed and easy-going attitude.