Superlative form of grotty: most grotty.(adjective)
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Use "grottiest" in a sentence
"They're made into Lex Luthor/Joker hybrids by the right wing in the US when in reality they're second cousins to the grottiest cosplayer at comic-con."
"They play a "night before christmas" show (i.e. something around the 20th of December) at one of the grottiest clubs in London (Apollo) every year."
"Go ahead and post the awfullest, grottiest, ancientest piece of juvenilia you still have a word processor that will open."