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The sound one makes when something is awesome enough to bring one joy to the point of intense sexual arousal.

Originates from the "so i herd u liek mudkips" meme, where in the original story a child is presented with a Mudkip Pokemon plush toy begins to hump it wildly while making an "EEEEEEENNNFFF!" noise.
Natalie Portman and Mila Kunis do it in The Black Swan? EENF!
by PerogiXW1 June 08, 2011