To try on is to put clothes on in the shop to see if they fit. Try on the jacket, try it on first, ask to try on the shoes.
Sizes vary so much between makers that trying something on is the only reliable test, and online returns are so costly partly because it cannot be done. In British slang, to try it on means to attempt something cheeky and see if you get away with it.
1phrasetry onTo put clothes on to see if they fit.
“Can I try on these jeans in a bigger size, please?”
try on is a testing.
Practised in Clothes & Fashion, Phrasal Verbs