Most probably your transcription is correct: もちあたぼうよ
It is divided as
もち - a contraction of もちろん
あたぼう - a slangy version of あたりまえ
よ - a particle.
Regarding よ, I'm not really sure which definition applies, but it may be 語調を整えたり、強めたりする意を表す. Essentially it does not affect the meaning.
On your question of how, colloquial expressions are often not covered in ...