When sandals were the footwear and the roads weren’t paved, feet got dirty and smelly and needed to be washed before eating else the meal couldn’t be enjoyed.
This unpleasant job was assigned to the lowliest servant.
When Jesus, who already shown he could raise the dead and walk on water, did this for his apostles at the Last Supper, it was rather mind-blowing for them.
“When sandals were the footwear…”
Just not with socks. That is a gave sin indeed.
Seriously, though, that is also what makes the gesture of the woman who washed Jesus’ feet and anointed them (Luke 7:36-50) so deep and meaningful. Simon the Pharisee had no clue.