Meaning of “Kamasu(かます)” / “Kamasou(かまそう)”

“Kamasu(かます)” means “kick ass” if the word is used in some rough situation.
In this case “Kamasu(かます)” is an abbreviation for “Buchikamasu(ぶちかます)”.
This is not a slang, but the word sounds rough or wild or violet.
This is a verb. So,
“Kamasita(かました)” is past tense, and “Kamasou(かまそう) means “Let’s kick ass!”.

Example Conversation

A: Finally this is the our last match.
B: Yeah, we just do our best.
A: Yeah! Just Kamasou(かまそう)!

A: いよいよ最後の試合だな。
B: そうだな、あとは全力でやるだけだな。
A: よし、いっちょかまそうぜ！