i-adjective
extremely vexing; damn annoying
Vulgar, emphatic expression of irritation or disgust. Used in casual, often rough speech.
あいつの態度はけったくそ悪い。
That guy's attitude is damn annoying.
この天気、けったくそ悪いな。
This weather is really pissing me off.
The exact derivation is uncertain. The first part けったくそ may be a sound-symbolic or emphatic formation, possibly related to けったい (odd, strange) in Kansai dialect, combined with くそ (shit) for vulgar intensification.