Slang term used to describe an older slang term of being "square" which was often used to indicate akwardness or someone who is completely odd. If you put the letter "L" and the number "7" close enoughtogether, the two characters create a shape resembling a square.
I got lost in the city and ended up in an unfamiliar neighborhood where I felt completely like an L 7.
L 7 was used in the 50's/60's to represent being "square" or old-fashioned or un-cool. (People would hold up their thumbs and forefingers to create a square shape as a non-verbal representation.)