A niner is not just what you call freshman at high school it can also be anyone at any age. Anybody who messes something up in their life is considered a niner.

Mercury retrograde is the act of taking a shit whilst mooning one or more human beings. Your day can typically being going well, according to your horoscope, until you are forced into witnessing Mercury retrograde. Performing Mercury retrograde will often speed the astrological cycle forward for the individual performing the Mercury retrograde, bringing about happy and fruitful times to come.

Word used in the NATO Alphabet(phonetic alphabet) to represent the number 9

slang for SanFrancisco 49ers.

The ultimate and unbeatable rack in beer pong. Sink the first cup and ask for a rerack: the niner in a power I formation. You can't lose, it's physics.

1. (among trekkies) A fan of the television program Star Trek: Deep Space Nine.
2. A Star Trek Fan who believes Deep Space Nine is the best Star Trek.

Niner is a rite of passage ceremony marking entrance or acceptance into a group or society in High School, required for grade nine students to be fully "accepted" into the student community.
Another word that could be used for this term is Initiation. However niner is a more specific term referring only to grade nine initiations.
This type of initiation is mostly done in school trips that require students to sleep over,(for example; Muskoka Woods camp) and at night around 1:00 to 3:00 AM, grade 11 or 12 students would wake up, and go around "initating" niners in their sleep.
This term is a new type of initation, started in Bishop Allen Catholic High School, in Toronto, Ontario, Canada.