The tricky bit here is n-t-g together. We do this by pronouncing the n as /ŋ/, like the ng in “sing”, using the back of your tongue, then begin to make /k/, but keep it in, stop briefly, and let it out as /g/.
Again, we have the glottal stop for t at the end, though this is optional. (See the notes on “come off it!”)
This idiom means “I don’t believe you!”. It’s very informal.