a shortened version of rating something, usually the looks of a girl. It started from somebody asking "what would you rate that girl, 1-10". ppl then shortened it to i rate it, meaning i would take the time to rate that because she is hot enough to catch someones attention. If somebody walks by unoticed because she is not hot
, she does not deserve to be rated, so nobody would "rate it"
Therefore: I rate it is a way to let somebody know that a particular girl is hot