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Thanks to the song "on my line" by bay area rapper Taj-He-Spitz,this phrase became popular among many listeners.It is used when you can tell a good looking lady is physically attracted to you at a party or get together,therefore persuing sexual relations with her.
"She on my line bro"

or you can refer to the lyrics,"Is that yo girl,damn she fine,i aint gon' lie but she on my line,she on my line"
by Dj Taco March 02, 2010