A "Splinter Cell" (in this context) is when you do something particularly sneaky/a physical activity that is a little unusual and difficult. I don't mean anything kinky, just like climbing a pipe that other people have only dared each other to climb, stealthily getting past someone to get something/do something. Like that.
These are also known as a "Hitman" or, more rarely, a "Solid Snake".
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One time, I was out on the driveway trying to fix my dad's old hack saw. Then I looked up and noticed that someone was trying to bust into my neighbor's car. So, I just took the hack saw blade, walked up behind him real quiet. Then, quickly I slipped the blade around the front of his throat and pulled on both ends. It didn't kill him, but it scared the shi.t out of him, and the guy was bleeding all over the goddamned place.