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A slightly derogatory, humourous name for metalcore, particularly simple metalcore with too many 1 open chord breakdowns.

LMAO is an obvious reference to the internet acronym for Laughing My Ass Off and 2-step is both a dance seen at hardcore shows (which looks like an un-co version of skanking) and the section of a hardcore song which encourages such dancing. Putting the two words together implies that 2-step is laughable.
"hahaha..listen to those breakdowns...fucking LMAO 2-step bullshit"

"LMAO 2-step! *parody of 2-step dancing ensues*"
by Leo2113 July 22, 2006