I think we complement each other well again. 
And checking my despised Duden on-line for the sake of a cite: A Bummel is an aimless stroll. Or a tour through many bars (that is new to me, I would have called it Keipentour or Saufgelage)
ETA: And checking further the etymology:
Herkunft ⓘ
aus dem Niederdeutschen, ursprünglich = hin und her schwanken (von der beim Ausschwingen „bum, bum“ läutenden Glocke)
Origin
From Low German, originally meaning to swing back and forth (from the bell ringing ‘bum, bum’ as it swings).
You know, I took many Kneipentouren in my life, but I never called them “Bummeln”, but rather “stumbling”… 
It seems it was Studentensprache in the XIX Century. I did not know either, but drinking is allegedly the reason they took so long to finish their education. I can relate to that.
Could have been the reason why it took me 9 semesters to get my diploma instead of 7 semesters of Regelstudienzeit.
Just two semester over Regelstudienzeit is not Bummeln. In the '80s in Berlin, Sir, it would have been called overachieving (see → Streber).
Yeah, (West) Berlin in the 80s was a very special place. Half of your male students were only matriculated there to avoid military service. 
But please don’t ever call me a Streber again! 
(worst insult of my school days)
ETA: as for “Bummelstudenten”, I had student friends in Dortmund in the 90s, and all of them later had fine careers, but back then there was one guy that stood out on every party, because he was 15 years older than the rest. That was the record breaking long time student of Dortmund, he finally graduated after more than 40 semesters. I have heard that he’s dead now, and I’m sure his education lasted much longer than his professional career.
That is true. But have you ever thought about what that meant in practice? Female students and half of the male students had a great time, and the other half of male students… well, some went to East Berlin, were they were kings for a day with 25 West Mark (aka Deutsche Mark) Zwangsumtausch (weird that there is no English wikipage for the 25 Deutsch Mark you had to exchange 1:1 for worthless East Mark).
My apologies. Will never happen again.
See what you’ve done? You got us started.
But you’ve not answered my question! is it cognate to “bum”?
Not in Hochdeutsch, but perhaps – probably not – in Plattdeutsch is the best I and my google-fu can offer.
ETA: Pleonast’s google-fu is something to learn from. See next post.
Interesting. “Bum” to mean your rump is not cognate, but “bum” to mean a hobo is cognate. The wonders of the English language.
The trick of the English language is to steal from everyone!
I was looking for the origin of the German bummeln, when I should have looked at the origin of the English bum. English so often influences German today that I did not think of checking the other way around.
I forget where I read this, but it was something like “English doesn’t borrow words from other languages, it lures them into dark alleys, beats them up and steals their wallet and clothes.”
The problem with defending the purity of the English language is that English is about as pure as a cribhouse whore. We don’t just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and riffle their pockets for new vocabulary.
https://en.wikipedia.org/wiki/James_Nicoll#"The_Purity_of_the_English_Language"
That’s it! I don’t recognize the entire quote, so I think what I saw was someone’s paraphrasing of it. Thanks for the full quote, it provides better context for the fragment I sort-of remembered.
I think Bill Bryson used that quote in his book about English, Mother Tongue.