The phrase "arm slightly" is correct and usable in written English.
It can be used in contexts where you are describing a small or subtle movement or adjustment of the arm.
Example: "To improve your posture, try to arm slightly when sitting at your desk."
Alternatives: "adjust your arm a bit" or "move your arm slightly."
One witness said Mr. Sharpton raised his arm, slightly deflecting the thrust, which slashed in just under the left clavicle, narrowly missing the lung and major blood vessels.
Twelve minutes into the overtime, Mellgren ran onto Riise's long pass, lifted her arm slightly toward the ball, let it bounce and fired a shot toward Mullinix.
Arm slightly raised in an enigmatic gesture, she sits for long minutes in the downstage left corner, the same spot in which she sat, arms slightly raised in an enigmatic gesture, for much of "Einstein on the Beach".
The obvious home advantage, though, was of a skater responding fully to the embrace of the crowd — at one point, Sotnikova flicked her arm slightly, to urge on the clapping, and, when the music stopped, was practically contorted with thankfulness and wonder.
The approach was anterior with the patient lying supine and the arm slightly externally rotated.
If Karl Marx had been born in the web age, he might have had to adapt his famous call to arm slightly - "Makers of the world unite".
His stride was deliberate, arms slightly bent to frame his rigid torso.
The Giants' defensive linemen will try to get their hands on the 49ers' linemen, arms slightly extended.
He raised his arms slightly and squinted at them, a carpenter troubled by a burl in the wood.
Pull your tummy in, keep your shoulders and chest facing forward, your arms slightly out to the side for extra balance.
The way she was standing, with her arms slightly raised, she might have been talking to the air or reciting, or playing statues, like a little girl.
