According to the Forgotten Realms Campaign Sourcebook ...
Many drow also communicate with a hand cant, a special sign language, very akin to the thieves cant used by most thieves guilds.A silent hand code used to communicate in the Underdark. Unlike other languages that must be heard to be comprehended Drow sign language must be seen to gather meaning. It has no alphabet and or written form.
Taken from Forgotten Realms Wikia ...
Drow Sign is a sign-language type tongue which uses hand motions and expressions. It is, as the name suggests, used mainly by the Drow elves. Humans and other goodly races often confuse these motions as spell conjurations as noted in the novel Sojourn. It is notoriously difficult for non-elves to master as they often lack the manual dexterity and slenderness of an elf's hands, even the incredibly dextrous Artemis Entreri had difficulty getting his hand to enunciate the movements properly and lacks the finesse of a Drow even years after learning the language.