Excusez moi..? Is there something wrong with the word? I thought penis already is the polite expression for dick, dong, wang, schlong, shaft or cock. Sorry, honest question - it would be fascinating if it's like... the other way around in English?
Or you mean something like "that thing down there" or "wiener" or... uhm... "willy"? Hm... I'd probably fail to properly order them all according to the whole world's combined idea of politeness. Is "weewee" a word? Nah, too complicated. Penis it is - plain, simple and everyone understands it ¯\_ (ツ)_/¯
And sorry if there's anything offensive in my list. Now I actually think about that too, but I just don't know any better.