Lightning bolt symbol, aka thunderbolt text emoticon is used to convey things that may be shocking either electrically, or in a metaphorical sense. Lightning emoticon is even used to convey “lightning fast speed”, as in Google Amp project that has a lightning bolt as it’s symbol. It’s hard to say how to correctly write ACDC band as text without a lightning symbol. Apperently the usual correct way is AC/DC but, of course, a lot of people text it as “ACDC”, or “AC DC” as a result. In ACDC the lighting bolt sign probably stands for the loud sound of lightning thunder clap. They even have a song called “Thunderstruck” with a memorable chant “Thunder”.