The idiom 'she's let herself go' provides a compelling case study in pragmatic displacement. Its meaning is entirely non-compositional, bearing no resemblance to the literal semantics of its components. One does not physically release oneself or cease moving. Instead, the phrase functions as a speech act, conveying a complex social judgment that is understood instantly.

The causative structure, 'to let oneself', implies a deliberate permission or failure to prevent something. In this idiom, what is 'let go' is not a physical restraint but a set of socially constructed standards of appearance and behaviour. The transition is from a literal causative action to a figurative resulting state: one of perceived social deviance. The utterance doesn't describe this state; it enacts a judgment upon it. This makes it a classic example of what can be termed 'idiomatic disparagement'.

The pragmatic weight of 'she's let herself go' hinges on presupposition. For the utterance to be felicitous, both speaker and listener must share an understanding of a previously 'acceptable' baseline. The phrase implicitly references a former state of being that met a particular aesthetic or behavioural norm. Without this shared, unspoken context, the idiom loses its force.

This leads to its function as a performative utterance of social sanction. The speaker is not merely stating a fact but performing an act of social evaluation and, often, exclusion. The conditions of satisfaction for this speech act are not truth-based (is it 'true' she has 'let herself go'?) but socially based. The utterance is 'satisfied' if it successfully communicates disapproval and reinforces a shared community standard.

The illocutionary force of the utterance is its primary function. It is not intended to inform but to evaluate. The perlocutionary effect, or the effect on the hearer, can range from agreement and reinforcement of in-group status to offence if the hearer is the subject or sympathetic to them. The intent behind the utterance is to align the speaker and listener in a shared judgment against a third party, thereby reinforcing social bonds and norms.

Ultimately, the phrase 'she's let herself go' demonstrates how language can operate far beyond its semantic components. The displacement from literal to idiomatic meaning is not a simple substitution but a complex pragmatic process. It is fuelled by shared social knowledge and serves as a tool for enforcing unspoken rules.