Plural of bullhorn; a portable loudspeaker or megaphone used to amplify a person's voice so it can be heard over a large area.
From 'bull' + 'horn', probably named after the funnel-shaped appearance resembling a bull's horn, or because it projects sound as forcefully as a bull's horn. Popular in American English since the early-to-mid 20th century.
The megaphone was invented way back in 1878, but the 'bullhorn' brand name became so popular that people used it generically—kind of like how people say 'Kleenex' for any tissue!
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