I've never heard the word "rackless". You probably mean "reckless", which means "does something without thinking about the consequences."
"Tom drives too fast and he never wears a seat belt. He's so reckless."
A rack is some type of structure for holding things, almost like a shelf.
"Put the salt back on the spice rack."
It can also informally refer to a woman's breast.
"She has a big rack."