Penis — literally “horseradish”. Хрен can replace хер or хуй in 90% of the expressions. The word хрен is not very strong, хер is rude and хуй is very vulgar. This gives the opportunity to express one idea with different degrees of rudeness.
Наш хрен самый хреновый хрен в мире…
Our horseradish is the most appalling horseradish in the world…