A word made up by
people uneducated in the ways of etymology. NOT THE PLURAL OF PENIS.
The rule with aplural form with an I is derived from
latin and is only applicable when the word
ends in -US, and has no relevance whatsoever when the word ends in -IS.
When a word ends in -IS, the correct plural form is, in fact, -ISES.
(US-I)
The Octopus was languid.
The
Octopi were languid.
The radius was 5cm
long.
The radii were both of the same length.
The Hippopotamus was huge.
All the hippopotami were huge.
(IS-ISES)
Within most species the penis is used for reproduction.
The correct plural or penis is penises, not penii.
The
White Ibis is my favourite of all the Ibises.