Someone who commonly sips from a straw, often protruding from a beverage of various liquids.
One who sips sinisterly is considered extremely dangerous, take caution when approaching one in the wild.
man 1: dude, what is that thing?
man 2: i don't know, it looks pretty sinister
man 1: what is it sipping?!?
man 2: oh my word, it's a sinister sipper! run!!