Which term refers to a lack of moral discipline or self-restraint, especially in a sexual context?

The term that specifically refers to a lack of moral discipline or self-restraint, particularly in a sexual context, is "licentious." This word is often used to describe behavior that disregards societal norms and moral boundaries regarding sexuality and often implies an element of excess or promiscuity.

In contrast, "indulgent" suggests a tendency to allow oneself to enjoy pleasures or desires, but it does not inherently carry the same moral implications as licentious. "Lamentable" refers to something that is unfortunate or regrettable, rather than a description of behavior. Lastly, "inquisitive" relates to a strong desire to learn or ask questions, which is entirely unrelated to moral discipline or self-restraint. Therefore, the best fit for the definition given is licentious.