快: why does "fast" also mean "happy"? The character 快 is usually used for the meaning of "fast", but it is also used in the sense of "pleasant" or "happy" - for example, in the words 痛快, 爽快. It seems that both meanings are quite ancient.

According to the morphology, you'd expect this character to originally have the "happy" meaning - it has a 忄 radical - but the "fast" meaning is also very ancient, e.g.


  此马虽快,然力薄不堪苦行。——《晋书·王湛传》


How did this character pick up two meanings, which are not quite related?

And yes, 快 also has other meanings.
Not a very satisfying answer but anyway:

說文解字


  喜也。从心夬聲。苦夬切


說文解字注


  喜也。引申之義爲疾速。俗字作駃。从心。夬聲。苦夬切。十五部。


Notice the 引申之義爲疾速 from above, happiness extended to mean fast.

I don't know exactly the mentality of 喜 -&gt; 引申 -&gt; 疾速 but apparently people did think they were related at one point in time.

Maybe it has to do with 俗字作駃

大汉语词典


  駃 kuài［1］快马。“参见駃马”。［2］同“快”。


Must have been some mix-up somewhere.