Bù le 不了: So Simple, So Unknown to Foreigners

I'm not really sure why most of us laowai don't know about this little phrase. It's certainly not because it's too complicated. Maybe it's just so short we don't realize we've heard it. I've never seen any books or dictionaries that explain it, so I'll try:

Bù le 不了 = No thanks (I don't want to do something)

It's not really "no thanks," because there's no xiè 谢 in there, but it does mean "no" and I get the feeling it's used in situations where we would say "no thanks."Apparently, it's best used between friends or someone you can speak informally with rather than shop owners or your boss. Here are the two situations in which I've heard it used:1. A friend on her balcony waving to a friend walking past:

A: Nǐ yào bú yào shàng lái hē chá? 你要不要上来喝茶?Do you want to come up and have some tea?

B: Bù le. Wǒ huí qù le. 不了. 我回去了.No thanks. I'm going home.

As I understand it, in this situation "bù le" 不了 is short for "bú yòng le" 不用了 meaning "that's not necessary." But that explanation doesn't really help me because if I were inviting you for tea, I wouldn't assume it was necessary. Again, that's why I translate it as "no thanks."2. Two friends (A and B) chatting at a sidewalk cafe. A third friend (C) walks past.

A: Nǐ hǎo! Gēn wǒmen yìqǐ chīfàn ba. 你好！跟我们一起吃饭吧.Hi! Eat with us.

C: Bù le. Wǒ yǐjīng chī le. 不了. 我已经吃了.No thanks. I already ate.

Notice in both situations they are friends. Also notice that "bù le" 不了 is immediately followed by a jièkǒu 借口 of some sort.Does anyone else have some insight into this little nugget? Please enlighten us.