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2.
It's getting the horn on , ie, an erection. Certainly I heard and used this phrase in the around 1960 - common UK working class phrase.
"I've got the horn." "She gives me the horn." "She gives me a horn on." (horn-on?)
by Charlie June 11, 2004
 
1.
To use the telephone.
Hey Simpson, get on the horn and tell accounting we need more TPS reports.
by famousramos June 11, 2004