adverb
Slang intensifier used before adjectives or verbs, similar to 'super' or 'hella'. Often carries a rough or emphatic tone.
今日はばちくそ暑いな。
It's super hot today.
あの映画、ばちくそ面白かった。
That movie was extremely interesting.
Likely a compound of ばち (from 罰, 'punishment') and くそ ('shit'), used as an intensifying slang expression. The exact origin is uncertain.