What's the "een" in Halloween?
word-forming element meaning "highest, topmost, at the extremities," before vowels, acr-, from Greek akro- "pertaining to an end, extreme," comb. form of akros "at the end, at the top" (see acrid).
acro- or acr-pref.
Top; summit; height: acrophobia.
Tip; beginning: acrosome.
Extremity of the body: acroparalysis.