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.
Kid 1: Dude, he just drew a bunch of penii!
Kid 2: What the hell is “Penii?”
Kid 1: Its the plural form of penis. Duh
Kid 2: The plural word is Penises. Idiot!
A false plural for "penis" made up by the same smug pseudo-intellectual dorks who say "boxen" and "ignorami" and "virii". Also, most of them wear suspenders use UNIX (variants of which they call *NIXen).
Guy 1: "Some ignorami stuck their penii in my Linux boxen and now they're borken! This is worse than Winblows boxen with all their virii!"
Passer-by: "Who are you talking to?"