Probably onomatopoeic, according to the Oxford English Dictionary. <span class="searchmatch">skedonk</span> (plural <span class="searchmatch">skedonks</span>) (South Africa) A banger; an old, battered motor car. Dokkens...
<span class="searchmatch">skedonks</span> plural of <span class="searchmatch">skedonk</span>...