i-adjective
terrific; staggering; incredible
Slang intensifier used as an i-adjective. Emphasizes that something is extreme or amazing. Often used in casual, youthful speech.
See also: ものすごい
あの映画、ものっそい面白かった!
That movie was incredibly interesting!
今日はものっそい暑いね。
It's terrifically hot today, isn't it?
adverb
extremely; incredibly; tremendously
Slang adverbial use, modifying verbs or adjectives to mean 'to an extreme degree'. Interchangeable with the adjective form in many contexts.
昨日はものっそい疲れた。
I was incredibly tired yesterday.
彼はものっそい速く走る。
He runs tremendously fast.