What does it mean?

I read, a lot. Often, I encounter a word that I do not know the exact meaning but can surmise the meaning from the text.  I know spendthrift meaning is a person that spends excessively.  I know that thrifty means to spend responsibly. So, why is thrift in spendthrift the opposite of thrift in thrifty?

Thrift...

1. economical management; economy.

2. Also called thrift′ institu`tion. a savings and loan association, savings bank, or credit union.

3. any alpine and maritime plant belonging to the genus Armeria, of the leadwort family, having pink or white flowers, esp. A. maritima, noted for vigorous growth.

4. vigorous growth.

5. Obs. prosperity.

Thrifty...

(of a person or their behavior) using money and other resources carefully and not wastefully:

Spendthirft...

a person who spends money in an extravagant, irresponsible way:

So, why the opposite meaning from two seemingly similar words? The web knows!

"Spendthrift" derives from an obsolete sense of the word "thrift" to mean prosperity rather than frugality, so a "spendthrift" is one who has spent their prosperity

Now, so do you. And, I...