'Having you here, Anna, firmly establishes what I've long suspected that we really are the most desperate lot of underachievers.' This is from Notting Hill right? In this context, Anna is a famous actress and those at the dinner party see her as an incredibly high achiever (someone who has accomplished much and is doing very well in her life). He is saying that having her there makes him realise that he and his friends are the opposite of that and that they have not accomplished much in life. This is an example of British self-deprecating humour. Later in the scene, he challenges his friends to see who's life is the worst and they are surprised to find that Anna's life is not as happy as they expected.

The speaker is telling Anna that because she is here, he knows that they are both never going to accomplish as much as they could. firmly establishes = proves what I've long suspected = something that I have thought for a long time desperate = it can mean "in great need," or "hopeless," but in some cases, such as this one, it can mean "unable to change." The adjective "hopeless" can be used the same way. For example, in the phrase "hopeless romantic" it means someone who will never become more practical and stop being so romantic. "underachiever" = Someone who chooses to accomplish less than they are capable of. (Lazy is USUALLY, a pretty good synonym, but not always.) So, a "desperate underachiever" is someone who doesn't want to accomplish very much, and will never become more motivated.

به نظر می رسد که شعر است ، و این مربوط به بینایی او در رابطه با آنا است ، او می گوید که هر دو کجا هستند ، یعنی در آن شرایط ، نشان می دهد که آنها یکسان هستند ، دو ناامید و ناموفق ، ناموفق شاید در جدایی و ناتوان بودن ، اگر تسلیم عشق شد. ممکن است بیش از یک تفسیر وجود داشته باشد ، من آن را از منظر شعر رمانتیک می بینم. it sounds like poetry, and it concerns his vision in relation to Anna, he says that where they are both, that is, in that situation, it shows that they are the same, two desperate and unsuccessful, unsuccessful perhaps in separating and were unable, if yielded to love. There may be more than one interpretation, I see it from the perspective of romantic poetry.

It means that he more or less realized that both of them are doing worse than they should be doing. Underachiever is someone who's doing worse than he should be doing. Being desperate might refer to the fact that they will do whatever they need to do (good or bad) to achieve something.