What's the "een" in Halloween?
mid-14c., houpen, partly imitative, partly from Old French houper "to cry out," also imitative. It is attested as an interjection from at least mid-15c. The noun is recorded from c.1600. Extended form whoopee is attested from 1845, originally American English; whoopee cushion is attested from 1960. Phrase whoop it up "create a disturbance" is recorded from 1884. Expression whoop-de-do is recorded from 1929. Whooping cough (1739) is now the prevalent spelling of hooping cough; whooping crane is recorded from 1791.
whoop (hōōp, hwōōp, wōōp)
The paroxysmal gasp characteristic of whooping cough.