about; roughly; generally; completely ~
およそ marks rough estimates and can also mean 'completely' in negative sentences.
およそ has two main uses. First, it indicates an approximate number or amount, similar to だいたい or やく, but it sounds more formal and literary. It is often used in writing or formal speech, especially with large or abstract quantities. For example, およそ百人 means 'about a hundred people'. This use is common in news, reports, and essays.
Second, およそ can combine with a negative verb to mean 'not at all' or 'completely not'. In this case it emphasizes that something is far from being the case, and it carries a strong, sometimes critical tone. For example, およそ不可能だ means 'completely impossible'. This use is also found in formal contexts.
In conversation, およそ is less common than だいたい or やく for estimates, and using it in casual speech may sound stiff. Be careful not to use およそ with a negative verb to mean 'about not doing something'—that is not the meaning.