A chunk of ice or hard slush, usually dark brown, which is found in the middle of, or alongside, the road in winter, having fallen off a passing car. It builds up like a stalactite behind the wheel from dirty road slush being thrown up there and sticking. Also known in some quarters as a slurd.
Dear Uncle Pete... Weather was a bit warmer today so car boogers were in evidence everywhere on the roads... kids were dodging cars and running out to kick them apart on their way home from school.... p.s. Mom's hemorrhoids are acting up again.