<human>: Could you distill the essence of the text into 1-3 sentences?

this happened years ago but hey i recently joined reddit. I was around 15 and new to America so I was quite unfamiliar with sexual references and American slangs. In my class we were actually discussing public transport problems relating to young people not giving up their seat for the older travelers. 

I stood up to share an anecdote which involved violence on a train which happened when an old lady started beating a young guy for not standing up for her. I proceeded to say, "to make the guy leave, the granny started 'fisting' the poor guy".....

The entire class burst out laughing and the teacher was shocked that I said that. I awkwardly asked whats so funny and that seemed to make it much worse. It was horrible given that i was new in school. I had no idea what fisting actually meant until after they told me; i thought it just meant to beat someone up.
<bot>: I put the image of a granny 'fisting' a guy on a train, in everyone;s mind by accident.