I heard the beauty salon specializes in slimming.
I kept getting massages for about a year, but I didn't have any slimming results.
The person caught by security camera was a slim guy.
A thin body type is called lean or slim. These days this is usually used in the context of slimming salons and massages you can get that may or may not work.
This is a jukugo word that uses the on'yomi readings of the kanji. You've learned them already, so you should be able to read this on your own!