The term "delusional" or "delusional" itself is not inherently offensive, as it is a medical and psychological term used to describe a specific state of mind in which a person believes things that are not based in reality. It is commonly used in clinical settings to describe a symptom of certain mental disorders, such as schizophrenia and psychotic disorders.
However, like many medical terms, it can become offensive when used inappropriately or as an insult towards someone in a derogatory manner. Using the term to belittle or demean someone's beliefs or opinions, especially in a non-clinical context, can be offensive and hurtful.
As with any language, it's essential to use words with sensitivity and respect towards others. If you suspect that using the term might be perceived as offensive in a particular context or towards a specific individual, it's better to find alternative, less potentially hurtful ways to express your thoughts or concerns.