The word "impotent," in German as in English, has two meanings: powerless in general and sexually incapable. In English there are hundreds of slang terms for this, but I've been unable to find any in German. The word "impotent" itself is too technical for my purpose.
wiki DE Tote Hose
"Jugendsprache: Schwunglosigkeit, auch männliche Impotenz"
The OP leaves it open (unless we read only the headline) if he thinks of impotent in terms of sexual activity or impotent in terms of general inability to act in what area ever.
For the latter, terms in use are
Note that these are explicitely not being understood as refering to sexual activities.
Also there are terms to accuse people of not being physically impressive:
Or to behave non-masculinely *
For direct replacements of impotent as an adverb:
- Whatever "masculine" means. This is an open discussion.
An euphemistic expression is
nicht mehr können [er kann nicht mehr]
Only used when addressing the elderly: