Music that is extremely unpleasant on the ears, causing intense displeasure and shouldn't be described as real music i.e. Nickleback. A portmanteau of the adjective British slang usage of gash, meaning something rubbish, worthless or very bad and the suffix "-core" derived from the word hardcore, used here to add extremism. It is satire of the overuse of -core to create new musical sub-genres.
"What on earth are you listening to? That music is gashcore mate! Put something better on."
"Don't let Joe pick the music, he'll put gashcore on"
"Don't let Joe pick the music, he'll put gashcore on"
by TheophrastusBombastus May 22, 2015