(SPOILERS FOR RDR2)
In one of the endings for Red Dead Redemption 2 (Specifically helping John with Low Honor) Arthur gets into a fight and is eventually shot by a former member of the gang, Micah. During this sequence, Micah will say *Damn you!" and Arthur will reply with *Damn us both!". The way Arthur says "Damn us both" sounds like "Tennis Boat" which eventually became a meme.
Micah: "You're not better than me, Morgan!"
Arthur: "Whatever you say, you fool!"
Micah: "Damn you!"
Arthur: "TENNISBOAT!"
*Arthur dies*
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.