Erectile dysfunction.

Sorry for the tangent, but why does this word get a "y" in the "dys", when any other "dis" word is spelled with an "i": disturb, disrespect, discombobulate, disconcerting, etc.

Dys means "abnormal" or "bad" or even "impaired" Dis means "without" or "lacking"

Dysfunction in this case means 'impaired function' as opposed to 'lacking function'.

What bitops and noahc said. See http://www.etymonline.com/index.php?term=dys-&allowed_in... and http://www.etymonline.com/index.php?term=dis-&allowed_in....

Different root - think dysphoria.