The Brief: Copystriking refers to filing a copyright claim with YouTube, which could lead to a copyright strike.
Copystrike is shorthand for a YouTube copyright strike. Copystriking is the informal name of the act of reporting content to YouTube that may be taken down for copyright violation in a Content ID claim.
The term was frequently used in reference to the dispute between PewDiePie and Twitch streamer Alinity. After PewDiePie called her a “Twitch THOT” in one of his videos, Alinity reportedly profited from copy striking, including from filing a copyright claim against PewDiePie for reacting to footage from her channel in a video.