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Old English gelpan (Anglian), gielpan (West Saxon) "to boast," from Proto-Germanic *gelpanan (cf. Old Saxon galpon, Old Norse gjalpa "to yelp," Old Norse gjalp "boasting," Old High German gelph "outcry"), from PIE root *ghel- "to cry out" (see yell). Related: Yelped; yelping.
Old English gielp "boasting," from source of yelp (v.). Meaning "quick, sharp bark or cry" is attested from c.1500.
The screaming and wavering warning signal used on police cars, ambulances, and other emergency vehicles: two police cars going north with yelpers wide open (1970s+)