also: めたくそ · めったくそ
adverb
Colloquial intensifier similar to めちゃめちゃ or めちゃくちゃ, but with a slightly rougher or more emphatic feel. Often used in casual speech among friends.
See also: めちゃめちゃ
昨日のテスト、めちゃくそ難しかった。
Yesterday's test was awfully difficult.
あの店のラーメン、めちゃくそうまいんだよ。
The ramen at that shop is super delicious, you know.
Can mean 'very' or 'messy'; めちゃくそ is only used as an intensifier and sounds more casual.
Likely a variation of めちゃくちゃ, with the second element replaced by くそ ('crap') for added emphasis and a rougher tone. The exact derivation is uncertain.