Two fingers

When I was very young my mother told me to clean some part of my shoe, the heel maybe. I asked her what she meant. She said I knew very well and not to be impertinent. But I really didn't know...

When I started in 1st form at secondary school I was only aware of one other Christian and he was in the 2nd form. When I saw him use the two fingers sign, clearly in a derogatory way, I asked him what it meant. I had seen other kids using the sign and thought that he, at least, would not mock me for not knowing. But he wouldn't say - he said that everyone knew what it meant. But I didn't.

I assumed therefore that it was sexual, hence the reticence, and could only suppose it was meant to look like a woman's legs apart.  But then I found the same sign being used for Victory - which confused me. Apparently it depends not only on the hand position and context, but also the nationality: it is only rude in the British Isle although my etymology may not be quite right see here.

My point being - why do people assume that the other person knows what they are talking about?

I remember my father for his sayings. One of them was that, if you are with others and are not sure you understand what they are talking about, you will learn faster if you ask what they mean even if you think so doing will make you look stupid. I regret that I sometimes despise folk when they don't know what I consider everyone should know. Not good...