The etymology of "coleslaw"

Coleslaw came to us from Dutch via American English.  Dutch kool means “cabbage” (cf. German Kohl, known to most as the family name of Helmut Kohl, the Chancellor of Germany, 1982-1998), while Dutch sla is the common colloquial form of salade “salad.”  Thus, the etymon is koolsla. 

It is unclear why the second element of coleslaw rhymes with haw, paw, raw, rather than with spa.  Words like spa are rare in Modern English; outside baa-baa, blah-blah-blah, bah and their ilk they are exotic borrowings (ah sounds more natural in the unstressed syllables of names like Sarah, Hannah, and so forth, including Monty Python’s Peckinpah).  The change from ah to aw may have been the result of the word’s domestication.  Or do we owe the shape of the vowel to the Midwesterners, in whose speech Shaw is indistinguishable from Shah?

From the Oxford University Press blog.