Grammar JLPT Level N4 - Lesson 14

大体(だいたい) is an adverb that is used to highlight the 'main point' of something in Japanese, and therefore has several different nuances. 大体(だいたい) will regularly be translated as 'mostly', 'generally', or 'for the most part'.
As with other adverbs that highlight amounts, 大体(だいたい) can be used to modify entire phrases, or can be used directly before a noun as 大体(だいたい)の + noun.

• だいたいでいいから何時(なんじ)くらいに着(つ)くか教(おし)えて。
  Since a general time is okay, can you let me know around when you will arrive?
• 水曜日(すいようび)はだいたい5時(じ)に起(お)きています。
  On Wednesdays, I generally wake up at 5 am.
• だいたいの人(ひと)たち薔薇(ばら)という漢字(かんじ)を書(か)けません。
  Most people do not know how to write the kanji 'bara'. (薔薇(ばら) is the kanji for 'rose')

In a similar way to そもそも, a noun/adverb which means 'in the first place', 大体(だいたい) can also have this meaning. This nuance is frequently quite obvious, as the rest of the sentence will highlight a desired outcome that unfortunately did not happen.

• だいたいなんでお前(まえ)がここにいるの？
  Why are you here in the first place? (No one wants you here)

Although these 2 primary uses of 大体(だいたい) may seem quite different, they are both just expressing what is 'essentially' correct, most common, the original, etc.