What is the origin of the idiom "let oneself go" and when to use it?

The origin of this phrase is not entirely clear, but it may have originated in the early 20th century, when it became fashionable for people to be more concerned about their appearance and personal hygiene. It was considered improper to appear disheveled or unkempt in public, and "letting oneself go" was seen as a sign of laziness or lack of self-respect. It is typically used in a negative or critical manner to comment on a decline in someone's overall well-being or appearance. It can be used in various situations, such as when discussing someone's appearance, health, grooming, or lifestyle choices.