Start scumming is a simple abuse that involves immediately quitting characters that do not meet some arbitrary criteria. Wizards are most often the victim of this scumming behavior, due to their useful starting magical equipment. For example, a player may startscum Wizards looking for a very good starting spell (such as identify or magic missile). Every role is startscummed to some extent, even those who receive no random equipment, because sometimes players demand unusually high starting stats.
On nethack.alt.org, startscumming has been defined as a quit or escape with less than a thousand points. This is by no means a perfect definition, as players sometimes commit suicide instead of quitting, and sometimes players quit or escape with no intention of scumming (such as when they accidentally choose the wrong role).
Many NAO scripts that generate stats based on the logfile (including Rodney's death announcements) ignore startscums as they have defined them. This is partly because about 34% of games on nethack.alt.org fall under their definition of startscumming; more than half of those scums are Wizards.