colloquial language - Why can だって have so many meanings?

According to jisho.org, だって has these meanings:

1. after all; because


2. but


3. even


4. too; as well; also



5. they say; I hear; you mean

In my opinion, "because" and "but", and "as well" are completely different things. I understand that the actual meaning of だって is inferred by context, but having so many distinct meanings is kind of weird.

I know that だって is short for だという, which means roughly "[some pronoun] say". That is the meaning number 5 of だって.

How does "[some pronoun] say" turn into "because", "but" and "even" etc?

Answer

1 and 2 are conjunctive だって used at the beginning of a sentence.

• 買わなかったよ。だって、高かったんだ。
  
  I didn't buy it. [After all / Because] it was expensive.


• 「買えって言ったでしょう？」「だって、高かったんだ。」
  "Didn't I tell you to buy it?" "But it was expensive!"

This kind of だって is used to argue back, or to provide further explanation to convince someone. It's translated as "but" or "because" depending on the previous context. If you're not sure, something like "you know" or "well" should work.

3 and 4 are mid-sentence だって used after a noun to emphasize it.

• 君だってできる。 Even you can do it. / You can do it, too.



• お前だって見ただろう？ You saw it, too, didn't you?

Definition 5 describes sentence-end だって. Sometimes it can sound mildly accusatory. In this case って is just a colloquial variant of the quotative と.

明日は雨だって。 [They say / I heard] it will rain tomorrow.

I don't know how だって is related to the verb 言う etymologically. だって is short enough and I don't feel it's worth analyzing this word too much.