“Don't try to act tough.”
Your brother cut his finger while cutting an onion. The cut looks serious, but he says that it's "just a scratch". You know that it hurts, and you want him to treat the cut correctly. You say this.
Don't try to act tough.
To "act tough" means to do things to make people think that you are tough. You can use "act ___" with other positive adjectives that describe people:
"Tough" describes something that can withstand a lot of use or damage, but keep working. A "tough" person doesn't complain when they get injured, and can work for a long time without stopping. Objects can also be tough. For example:
a tough truck
a tough camera case
You say this when someone is pretending to be strong or tough, but you know that they really aren't that way.