Samen is originally German for "seed".
Semen originally means "seed (of a plant)" in German, but in Japanese it is practically used as another name for semen. However, it has become famous simply by paraphrasing it into a foreign language. It is no longer used as slang. For some reason, this is more commonly used than the English spelling. Is it because German has a good sense of language (for Japanese)? The German word "Samen" also has the meaning of "semen", so it's not a mistake to use it in Japanese. If semen were to be expressed in jargon, it would be desirable to use "Calpis," "Kefir," "Yogurt," and "Milk." See the semen section for details.