1853, Charles Fenno Hoffman, Timothy Flint, Lewis Gaylord Clark, Kinahan Cornwallis, and John Holmes Agnew (eds.), , page 460,"Then the peepers begin on a high key, with a singularly sweet and lucid voice, somewhere betwixt a silver-whistle and a glass-bell, smacking little of the mid: 'Eep!-eep-eep-eep! ee ee-ee! eepee! eepee-peepee! peep-eep! eepepee! eepepee! eepepee!' accompanied by a few trills long continued..."
1962, Jet Screamer, The Jetsons, "Eep opp ork ah ah! And that means 'I love you'!" (but, according to Elroy Jetson in the episode "A Date with Jet Screamer", he says Judy Jetson wrote it for him, "eep opp ork ah-ah" means "meet me tonight")http://www.tvacres.com/music_songs_jetsons.htm (Note: this reference is incorrect.)