This slang contraction appears to be too new for most dictionaries or Google NGrams. Since it's slang and new, I took at look at social networking and the web for ideas. More commonly used forms often lead to standard forms over time, all else being equal.
Urban Dictionary lists both cas and cazh, the former having more votes there.
Google Searches reveal that "cas dress" (6,000+ hits for me) is found much more often than "cazh dress" (50+ hits for me). "dress cas" is actually quite common, with 130,000+ hits for me compared to only 60 for "dress cazh".
Twitter Search shows that cazh is used, mostly by people asking how to abbreviate 'casual'. A cas search has far too many unrelated results to judge. "cas dress" has only 4 hits and "dress cas" only 1 applicable, but "cazh dress/dress cazh" and "cash dress/dress cash" have no related results.
Finally, a Google search of Facebook shows that "cas dress/dress cas" has a few hits (41 & 13 for me) but "cazh dress/dress cazh" have none.
I also did Twitter and Facebook searches for "sports cas" and "sports cazh" but there were no related results in either case.
Based on social networking results, I'd say that there is still no common way to spell this abbreviation - but if pressed, 'cas' seems to be slightly more common than 'cazh'. It seems that more people simply abbreviate the existing word "casual" than try for a slightly obscure phonetic spelling.
It is thought to have derived from Welsh and is often considered derogatory. Use renege or other wording instead.
Online Etymology Dictionary
Etymonline.com says of welch:
1857, racing slang, "to refuse or avoid payment of money laid as a bet," probably a disparaging use of the national name Welsh.
And of Welsh:
Among the English, Welsh was used disparagingly of inferior or substitute things, hence Welsh rabbit (1725), also perverted by folk-etymology as Welsh rarebit (1785).
Oxford English Dictionary
The OED says of the verb welsh or welch:
Origin uncertain; perhaps < Welsh adj., on account of alleged dishonesty of Welsh people (see note). Earlier currency is probably implied by welsher n.1, welshing n., and welshing adj.
Sometimes considered offensive in view of the conjectured connection with Welsh people.
Their first quotation meaning to renege on a betting debt is from an 1860 Racing Times:
The plaintiff denied that he had ever..‘welched’ a man named Williams at Worcester in 1854.
Their first quotation of noun welsher, a bookmaker who refuses to pay, is from an 1852 Racing Times:
One of the above fraternity [sc. betting impostors] was observed following his calling, by a former victim... The ‘Welsher’ sneaked off to another corner of the field.
Their first for noun welshing is from an 1854 Era:
The subterfuge and welching of the betting enclosure.
BBC
But it is still used, often by politicians, including the BBC itself. Occasionally they apologise. The BBC reported in February 2012 that Education secretary Michael Gove apologised for saying he'd "welshed on the deal" in the House of Commons, and 'Bill Clinton apologised to Republicans in 1995 for calling them "Welshers"'.
Best Answer
Nut is slang for head. And nut case means head case; i.e, mind/brain injury/illness.
It's a pretty obvious metaphor: nuts and human heads have breakable covers, are spherical living things, and are filled with other important living things (some of which may even resemble one another -- compare walnut meats with brain hemispheres visually, for instance).
Nut case, in turn, results in the adjective nuts, meaning 'crazy'. Then, by comparison, crazy about
Xbecomes nuts aboutX; and someone who is nuts aboutXis of course anXnut.