I'm sorry to report to you that you have incorrectly pluralised the word "penis".
You see, a word that ends in "is" in it's singular form, goes in the same plural suffix category as that of words like "Crisis", which gets pluralised to "Crises".
If the plural
were Penii, the singular form would have to be "Penius". Having 2 "i"s as the end in its plural form, like "radii" to "radius", the word in it's singular form would have to end in "ius", as, when pluralised, the "us" is replaced by only 1 "i". You will notice that words like "octopus" get pluralised to "Octopi", with only 1 "i" at the end.
These systems of pluralisation, however, are not as consistent or black and white as one might think, as some words do not abide by them.
If you would like to follow
this link to Wikipedia, you will learn a little more about the origins of pluralisation from foreign and historic languages. I hope you enjoy it.
So, in conclusion, you made 3 mistakes:
- You presumed the word "penis" follows it's 'logical' system of pluralisation.
- Presuming the word "penis" does, in fact, use it's 'logical' system of pluralisation, you used the wrong system.
- Assuming you used the right system, you incorrectly doubled up the suffix.