And how did "spendthrift" come to mean someone careless with money?

We've dealt with this before, albeit briefly :-)

http://wordsmith.org/board/showthreaded.pl?Cat=&Board= wordplay&Number=39907&page=&view=&sb=&vc=1#Post39907

(left in this form to avoid broadening the thread - but can be reconstituted by cut-and-paste-ing)