PTDM stands for "person that doesn't matter" (the plural, PDTMs, stands for "people that don't matter"). The acronmyn is one typically employed by young persons (that is, people under the age of 18, especially high school students). Nevertheless, "PTDMs" may be used by people of any age to refer to someone anyone who is generally lame, boring, or judgemental.
by billymorgan August 29, 2007