Snuff is a type of smokeless tobacco that is ground up to be ingested either via the nose or the mouth. It is often used for instances where people aren't allowed to smoke.
Strangelove and Big Boss
Dr. Strangelove, utilized snuff due to her work with AIs, as cigar(ette) smoke would ruin the precision equipment. Although Big Boss rarely used snuff, he once ingested it at Strangelove's suggestion before entering her AI Lab.
Paz Ortega Andrade
The Cipher agent Pacifica Ocean, better known as Paz Ortega Andrade, utilized snuff during her stay at Mother Base, as she had to keep up the appearance of being an innocent schoolgirl for her mission. She specifically used the kind where she placed it directly beneath the gums and wasn't used to it, which gave her a minor quirk where she places her finger on her lip to keep it settled, something Kazuhira Miller and Big Boss later deduced after she was exposed as a spy. When revealing her true loyalties by hijacking Metal Gear ZEKE, she proceeded to admit that she used snuff tobacco and proceeded to spit it out, claiming she hated it.