Right after he started communicating with Earth, Mark was warned (by Mission Control) that he needed to to stop cussing because the whole world could hear him.
What did he type as a response?
In the source novel it was the text emoticon for breasts.
[12:04] JPL: We’ll get botanists in to ask detailed questions and double-check your work. Your life is at stake, so we want to be sure. Sol 900 is great news. It’ll give us a lot more time to get the supply mission together. Also, please watch your language. Everything you type is being broadcast live all over the world.
[12:15] WATNEY: Look! A pair of boobs! -> ( . Y . )
In Drew Goddard's earlier version of the script it was a lot ruder.
The whole room waits patiently. Tim reads the response.
TIM: He says... “They don’t know I’m alive? What the--” (hesitates) “What the... f-word... f-word in gerund form... f-word again... is wrong with you... f-words.”
Venkat: Mark, please watch your language. Everything you type is being broadcast live all over the world
Mark’s eyes narrow as he types his response. Hits ENTER
The group reads Mark's response. They go PALE. Oh, Jesus. Venkat hangs his head. Tim tries really hard not to smile and fails.
Teddy is on the phone as Mitch approaches.
TEDDY (INTO PHONE): Yes sir... he's under a tremendous amount of stress... I understand. We're working on it. Thank you, sir.
Teddy hangs up. Glances at Mitch.
TEDDY (CONT'D): I just had to explain to the President of the United States what a “bureaucratic felcher” is.
1 down vote In the novel Mark writes "Look! A pair of boobs -> ( .Y . )"
Although in the original serial posted to Andy's website it was something about felching (don't look that up please) and a blooper scene leads me to believe they almost went with that in the movie because it's a scene of Teddy explaining what felching is to Annie (again don't look it up if you don't already know) and then everyone laughs.
I did some research with the sources above and with some small infrinces I found he most likely typed in the movie "What the fucking god damn idea in gerund form. what the fuck is wrong with you bureaucratic felchers." this is gathered from the original script, current script, the book, and reports from fox to try and keep it pg-13.