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:
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.
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