Siepen

• Seifen, Siefen

From Middle Low German sîpe, from the verb sîpen (“to trickle”). Cognate with Middle High German sīfe, whence the alternative forms, and further with Dutch zijp (“canal, sewer”).

• IPA^(key): /ˈziːpən/

Siepen m or n (strong, genitive Siepens, plural Siepen)

1. (regional, chiefly in toponyms) a swampy gorge, a brook in a valley

• In official toponyms the regional distribution of the forms is roughly as follows:
  • Siepen in Low German and Low Franconian areas, chiefly the Bergisches Land and Sauerland.
  • Siefen in the Central German parts of North Rhine-Westphalia (except Siegerland).
  • Seifen in other Central German areas.
• Appellative use outside of purely local contexts is rare in Standard German, but it appears that the proper form Seifen is actually the least favoured, without doubt because of its homonymy with Seifen (“soaps”). German wikipedia uses Siepen as lemma, which we follow here.