Short explanation: Type ¯\\\_(ツ)\_/¯ to see ¯\_(ツ)_/¯.
Long expanation: Lemmy supports formatting, like _italic_ becomes italic. To stop this from happening, you can put a \ before it like \_; the \ isn't shown. This is why ¯\_(ツ)_/¯ becomes ¯_(ツ)_/¯. To show a \ you need an additional \ like so: \\, and to make sure _ is shown and not turned into italic, it too needs \. This is why ¯\\\_(ツ)\_/¯ becomes ¯\_(ツ)_/¯
Alternatively, you can just use the `` enclosure, used for single line code.
That is a "grave accent" or a "backtick", the key you will find on the left of the '1' key and under the 'Esc' key on a standard (ISO, maybe) 104/105 key qwerty keyboard.