The name Sernf (earlier also Sernft) is of pre-Germanic origin, either Celtic or an example of Old European hydronymy.
It derives from a hypothetical *Sarnivos, containing a PIE root *ser "to flow".[1]
The name of the Sernf river has received some attention in German online culture as the "fifth German word in -nf", popularized by Bastian Sick in his Spiegel Online blog.[2]
The word fünf "five" is the only genuinely German word with this ending, the others are early loanwords, including Hanf "hemp" (from kánnabis) and Senf "mustard" (from sinapis), and the toponym Genf "Geneva", from Genava.