He plans to 'thin' (out) the young carrots in the garden. [=to remove some of them so that there is more room between the remaining carrots]
His hair is a little 'thin' on top. = He's a little 'thin' on top. [=he does not have a lot of hair on the top of his head]
The bank's decision to raise rates could be 'the thin end of the wedge'. [=many other banks may also decide to raise rates]
(..) I'm on it. Oh no, I didn't say you could leave. War Machine, comin' at you, right a-... Okay, what? We gotta move out. Even I can tell the air is getting thin. You guys get to the boats... I'll sweep for stragglers, be right behind you. What about the core? I'll protect it. It's my job. (..)2015 Avengers: Age of Ultron