Questions & Answers about Saya tidak begitu lelah hari ini.
Begitu literally means so / that / like that, but in this context it works like “so” in “not so tired”.
- Saya tidak lelah hari ini = I am not tired today (sounds more absolute: not tired at all, or at least clearly not tired).
- Saya tidak begitu lelah hari ini = I’m not that tired today / I’m not so tired today (you are a bit tired, but not very).
So begitu softens the statement and makes it more like “not very” or “not particularly” tired.