Finger of suspicion, often abbreviated as FoS or fos is merely a way to tell everyone that they find some suspicious of being mafia.

It can be synonymous with a scum read.

Common Usage

Player1; I fos Player2

As a verb, it merely means "to suspect as mafia".

Player1: Out your foses
Player2: Okay, I fos Player3 and Player5

As a noun, it is a list of people that they suspect to be mafia.

Player1: Explain your fos(es)

Often, players will get asked to explain why they suspect a certain player.

LYLO, MYLO, WIFOM, Bus, Bandwagon, Blitz, Fakeclaim, Counter-claim, Power Roles, FoS