“You don't want to come across as too desperate.”
Your friend is a single guy who's not very good with women. He was able to get a girl's phone number and wants to call her the next day. You say this because you don't think he should call her that soon.
You don't want to come across as too desperate.
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This phrase is used to give someone advice about what not to do. It's used when you're an expert, or have some experience in a topic. So in the example above, the speaker sounds like he or she is experienced with dating or dealing with women.
The way that you "come across" to people is how they see you and think about you. This phrase is a way to explain what people think of you, which can be different from how you think of yourself. So although you may think of yourself as cool and sophisticated, you might come across as unfriendly instead.
You can follow "come across as" with a noun phrase that describes a person:
He came across as a really friendly and sweet guy at first.
Or you can follow it with an adjective phrase that describes someone's personality, like in the example above. Here's another example:
Do you think I come across to people as kind of boring?
But it sounds strange to just have a single-word adjective after "come across as", like "come across as boring" or "come across as needy".
A person who is "desperate" needs something so badly that they will do anything to get it. You can be desperate for many things, including money, food, attention, drugs, or love.
This word sounds negative, so you use it when warning or critizing someone.