nothing
nothin'
/ˈnʌθɪn/Elision
In real speech
What happens to the sounds
The -ing ending is said as -in: /ŋ/ becomes /n/ (“g-dropping”). Same with somethin', goin', doin'.
When to use it
Very common in speech and songs. The apostrophe spelling (nothin') is only for informal writing.
Casual → formal
Slang
nothin' / nada
Neutral
nothing
Formal
not anything
Sound notes
Elision: a sound disappears completely (often t, v, h or th).