The attitude connoted by the word <em>snicker </em>is condescending disrespect.
The word <em>snicker</em> is defined by the Merriam-Webster dictionary as "to laugh in a covert or partly suppressed manner."
There is something false and dishonest about a snicker; it gives an impression of sarcasm, contempt, or mockery. It can be interpreted as disrespectful and condescending.
This could correspond to the eternal Footman's (death's) attitude because in this poem, Prufrock is expressing his fear of mortality and his feeling of not being important in the eyes of death.
