Slag is a commonly used Cybertronian expletive. Slag is generally defined as "the impurities which result and are separated out when melting a metal or refining it from its ore", and following this definition, it is an insulting word to Transformers, who are made of metal. The first recorded usage of it was by Starscream (when he said "We should vaporize those Autobot fools and fuse their carcasses into slag!"), and later, its first usage as an insult by the mighty Galvatron ("Die, you worthless piece of slag!"), though these are only human records, and the word possibly originated as an insult long ago.

In Mashchester, the word also has come to mean "slut" in slang. Transformers who used this word while in Britain were promptly arrested and brought before the Prime Minister, who was Michael Proctor, Jr. at the time. Since these few incidents in Great Britain, Transformers have been banned from entering the country, and all Transformers that had been stationed there were quickly deported.

In a different incident, Starscream once crash-landed in a forest owned by Peklo. The Transformer was safe, or so he thought, until Peklo came to investigate the crash. Starscream then merely shouted "Eat slag!" and shot several trees, setting fire to the forest, before flying away. The word "slag" was then banned from Finland the following day, and to this day, Peklo still hates Starscream for destroying his forest.

Slag is also the name of a member of the Autobot sub-group, the Dinobots. He is a Triceratops with a foul attitude and aggressive speech, hence his name.